diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
index 94cda34a29d0..391c8cc214c0 100644
--- a/lib/matplotlib/axes/_axes.py
+++ b/lib/matplotlib/axes/_axes.py
@@ -1978,19 +1978,19 @@ def make_iterable(x):
             if len(edgecolor) < nbars:
                 edgecolor *= nbars
 
-        # FIXME: convert the following to proper input validation
-        # raising ValueError; don't use assert for this.
-        assert len(left) == nbars, ("incompatible sizes: argument 'left' must "
-                                    "be length %d or scalar" % nbars)
-        assert len(height) == nbars, ("incompatible sizes: argument 'height' "
-                                      "must be length %d or scalar" %
-                                      nbars)
-        assert len(width) == nbars, ("incompatible sizes: argument 'width' "
-                                     "must be length %d or scalar" %
-                                     nbars)
-        assert len(bottom) == nbars, ("incompatible sizes: argument 'bottom' "
-                                      "must be length %d or scalar" %
-                                      nbars)
+        # input validation
+        if len(left) != nbars:
+            raise ValueError("incompatible sizes: argument 'left' must "
+                             "be length %d or scalar" % nbars)
+        if len(height) != nbars:
+            raise ValueError("incompatible sizes: argument 'height' "
+                              "must be length %d or scalar" % nbars)
+        if len(width) != nbars:
+            raise ValueError("incompatible sizes: argument 'width' "
+                             "must be length %d or scalar" % nbars)
+        if len(bottom) != nbars:
+            raise ValueError("incompatible sizes: argument 'bottom' "
+                             "must be length %d or scalar" % nbars)
 
         patches = []
 
@@ -2428,8 +2428,10 @@ def pie(self, x, explode=None, labels=None, colors=None,
             labels = [''] * len(x)
         if explode is None:
             explode = [0] * len(x)
-        assert(len(x) == len(labels))
-        assert(len(x) == len(explode))
+        if len(x) != len(labels):
+            raise ValueError("'label' must be of length 'x'")
+        if len(x) != len(explode):
+            raise ValueError("'explode' must be of length 'x'")
         if colors is None:
             colors = ('b', 'g', 'r', 'c', 'm', 'y', 'k', 'w')
 
@@ -3686,8 +3688,9 @@ def scatter(self, x, y, s=20, c=None, marker='o', cmap=None, norm=None,
         collection.update(kwargs)
 
         if colors is None:
-            if norm is not None:
-                assert(isinstance(norm, mcolors.Normalize))
+            if norm is not None and not isinstance(norm, mcolors.Normalize):
+                msg = "'norm' must be an instance of 'mcolors.Normalize'"
+                raise ValueError(msg)
             collection.set_array(np.asarray(c))
             collection.set_cmap(cmap)
             collection.set_norm(norm)
@@ -4057,8 +4060,9 @@ def hexbin(self, x, y, C=None, gridsize=100, bins=None,
             bins = np.sort(bins)
             accum = bins.searchsorted(accum)
 
-        if norm is not None:
-            assert(isinstance(norm, mcolors.Normalize))
+        if norm is not None and not isinstance(norm, mcolors.Normalize):
+            msg = "'norm' must be an instance of 'mcolors.Normalize'"
+            raise ValueError(msg)
         collection.set_array(accum)
         collection.set_cmap(cmap)
         collection.set_norm(norm)
@@ -4673,8 +4677,9 @@ def imshow(self, X, cmap=None, norm=None, aspect=None,
         if not self._hold:
             self.cla()
 
-        if norm is not None:
-            assert(isinstance(norm, mcolors.Normalize))
+        if norm is not None and not isinstance(norm, mcolors.Normalize):
+            msg = "'norm' must be an instance of 'mcolors.Normalize'"
+            raise ValueError(msg)
         if aspect is None:
             aspect = rcParams['image.aspect']
         self.set_aspect(aspect)
@@ -5000,8 +5005,9 @@ def pcolor(self, *args, **kwargs):
 
         collection.set_alpha(alpha)
         collection.set_array(C)
-        if norm is not None:
-            assert(isinstance(norm, mcolors.Normalize))
+        if norm is not None and not isinstance(norm, mcolors.Normalize):
+            msg = "'norm' must be an instance of 'mcolors.Normalize'"
+            raise ValueError(msg)
         collection.set_cmap(cmap)
         collection.set_norm(norm)
         collection.set_clim(vmin, vmax)
@@ -5149,8 +5155,9 @@ def pcolormesh(self, *args, **kwargs):
             antialiased=antialiased, shading=shading, **kwargs)
         collection.set_alpha(alpha)
         collection.set_array(C)
-        if norm is not None:
-            assert(isinstance(norm, mcolors.Normalize))
+        if norm is not None and not isinstance(norm, mcolors.Normalize):
+            msg = "'norm' must be an instance of 'mcolors.Normalize'"
+            raise ValueError(msg)
         collection.set_cmap(cmap)
         collection.set_norm(norm)
         collection.set_clim(vmin, vmax)
@@ -5274,8 +5281,9 @@ def pcolorfast(self, *args, **kwargs):
         cmap = kwargs.pop('cmap', None)
         vmin = kwargs.pop('vmin', None)
         vmax = kwargs.pop('vmax', None)
-        if norm is not None:
-            assert(isinstance(norm, mcolors.Normalize))
+        if norm is not None and not isinstance(norm, mcolors.Normalize):
+            msg = "'norm' must be an instance of 'mcolors.Normalize'"
+            raise ValueError(msg)
 
         C = args[-1]
         nr, nc = C.shape
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index c8e5b3cbb955..9a97ebb83ca9 100644
--- a/lib/matplotlib/axes/_base.py
+++ b/lib/matplotlib/axes/_base.py
@@ -2104,7 +2104,10 @@ def draw_artist(self, a):
         caches the renderer.  It is used to efficiently update Axes
         data (axis ticks, labels, etc are not updated)
         """
-        assert self._cachedRenderer is not None
+        if self._cachedRenderer is None:
+            msg = ('draw_artist can only be used after an initial draw which'
+                   ' caches the render')
+            raise AttributeError(msg)
         a.draw(self._cachedRenderer)
 
     def redraw_in_frame(self):
@@ -2113,7 +2116,10 @@ def redraw_in_frame(self):
         caches the renderer.  It is used to efficiently update Axes
         data (axis ticks, labels, etc are not updated)
         """
-        assert self._cachedRenderer is not None
+        if self._cachedRenderer is None:
+            msg = ('redraw_in_frame can only be used after an initial draw'
+                   ' which caches the render')
+            raise AttributeError(msg)
         self.draw(self._cachedRenderer, inframe=True)
 
     def get_renderer_cache(self):
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
index dba6fd15f686..4aed76dc85b7 100644
--- a/lib/matplotlib/axis.py
+++ b/lib/matplotlib/axis.py
@@ -1728,11 +1728,13 @@ def set_label_position(self, position):
 
         ACCEPTS: [ 'top' | 'bottom' ]
         """
-        assert position == 'top' or position == 'bottom'
         if position == 'top':
             self.label.set_verticalalignment('baseline')
-        else:
+        elif position == 'bottom':
             self.label.set_verticalalignment('top')
+        else:
+            msg = "Position accepts only [ 'top' | 'bottom' ]"
+            raise ValueError(msg)
         self.label_position = position
 
     def _update_label_position(self, bboxes, bboxes2):
@@ -2032,13 +2034,15 @@ def set_label_position(self, position):
 
         ACCEPTS: [ 'left' | 'right' ]
         """
-        assert position == 'left' or position == 'right'
         self.label.set_rotation_mode('anchor')
         self.label.set_horizontalalignment('center')
         if position == 'left':
             self.label.set_verticalalignment('bottom')
-        else:
+        elif position == 'right':
             self.label.set_verticalalignment('top')
+        else:
+            msg = "Position accepts only [ 'left' | 'right' ]"
+            raise ValueError(msg)
         self.label_position = position
 
     def _update_label_position(self, bboxes, bboxes2):
@@ -2083,13 +2087,14 @@ def _update_offset_text_position(self, bboxes, bboxes2):
         )
 
     def set_offset_position(self, position):
-        assert position == 'left' or position == 'right'
-
         x, y = self.offsetText.get_position()
         if position == 'left':
             x = 0
-        else:
+        elif position == 'right':
             x = 1
+        else:
+            msg = "Position accepts only [ 'left' | 'right' ]"
+            raise ValueError(msg)
 
         self.offsetText.set_ha(position)
         self.offsetText.set_position((x, y))
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
index edcc81f95f89..bd2072f1f6f6 100644
--- a/lib/matplotlib/backend_bases.py
+++ b/lib/matplotlib/backend_bases.py
@@ -969,7 +969,11 @@ def set_clip_path(self, path):
         Set the clip path and transformation.  Path should be a
         :class:`~matplotlib.transforms.TransformedPath` instance.
         """
-        assert path is None or isinstance(path, transforms.TransformedPath)
+        if path is not None and not isinstance(path,
+                transforms.TransformedPath):
+            msg = ("Path should be a matplotlib.transforms.TransformedPath"
+                   "instance.")
+            raise ValueError(msg)
         self._clippath = path
 
     def set_dashes(self, dash_offset, dash_list):
diff --git a/lib/matplotlib/blocking_input.py b/lib/matplotlib/blocking_input.py
index 2ed22b918664..d7035221e8eb 100644
--- a/lib/matplotlib/blocking_input.py
+++ b/lib/matplotlib/blocking_input.py
@@ -30,6 +30,8 @@
 from matplotlib.cbook import is_sequence_of_strings
 import matplotlib.lines as mlines
 
+import warnings
+
 
 class BlockingInput(object):
     """
@@ -38,8 +40,8 @@ class BlockingInput(object):
     """
     def __init__(self, fig, eventslist=()):
         self.fig = fig
-        assert is_sequence_of_strings(
-            eventslist), "Requires a sequence of event name strings"
+        if not is_sequence_of_strings(eventslist):
+            raise ValueError("Requires a sequence of event name strings")
         self.eventslist = eventslist
 
     def on_event(self, event):
@@ -95,7 +97,8 @@ def __call__(self, n=1, timeout=30):
         Blocking call to retrieve n events
         """
 
-        assert isinstance(n, int), "Requires an integer argument"
+        if not isinstance(n, int):
+            raise ValueError("Requires an integer argument")
         self.n = n
 
         self.events = []
@@ -146,9 +149,9 @@ def post_event(self):
         """
         This will be called to process events
         """
-        assert len(self.events) > 0, "No events yet"
-
-        if self.events[-1].name == 'key_press_event':
+        if len(self.events) == 0:
+            warnings.warn("No events yet")
+        elif self.events[-1].name == 'key_press_event':
             self.key_event()
         else:
             self.mouse_event()
@@ -359,9 +362,10 @@ def post_event(self):
         """
         Determines if it is a key event
         """
-        assert len(self.events) > 0, "No events yet"
-
-        self.keyormouse = self.events[-1].name == 'key_press_event'
+        if len(self.events) == 0:
+            warnings.warn("No events yet")
+        else:
+            self.keyormouse = self.events[-1].name == 'key_press_event'
 
     def __call__(self, timeout=30):
         """
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index 4bceb7417d60..9d6a73f12c8f 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -857,10 +857,12 @@ def __init__(self, ax, *args, **kwargs):
         else:
             self.logscale = False
 
-        if self.origin is not None:
-            assert(self.origin in ['lower', 'upper', 'image'])
-        if self.extent is not None:
-            assert(len(self.extent) == 4)
+        if self.origin not in [None, 'lower', 'upper', 'image']:
+            raise ValueError("If given, *origin* must be one of [ 'lower' |"
+                             " 'upper' | 'image']")
+        if self.extent is not None and len(self.extent) != 4:
+            raise ValueError("If given, *extent* must be '[ *None* |"
+                             " (x0,x1,y0,y1) ]'")
         if self.colors is not None and cmap is not None:
             raise ValueError('Either colors or cmap must be None')
         if self.origin == 'image':
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
index 986870c78140..f14c8c8c64a6 100644
--- a/lib/matplotlib/figure.py
+++ b/lib/matplotlib/figure.py
@@ -872,7 +872,9 @@ def add_axes(self, *args, **kwargs):
 
         if isinstance(args[0], Axes):
             a = args[0]
-            assert(a.get_figure() is self)
+            if a.get_figure() is not self:
+                msg = "The Axes must have been created in the present figure"
+                raise ValueError(msg)
         else:
             rect = args[0]
             projection_class, kwargs, key = process_projection_requirements(
@@ -946,7 +948,10 @@ def add_subplot(self, *args, **kwargs):
         if isinstance(args[0], SubplotBase):
 
             a = args[0]
-            assert(a.get_figure() is self)
+            if a.get_figure() is not self:
+                msg = ("The Subplot must have been created in the present"
+                       " figure")
+                raise ValueError(msg)
             # make a key for the subplot (which includes the axes object id
             # in the hash)
             key = self._make_key(*args, **kwargs)
@@ -1104,7 +1109,10 @@ def draw_artist(self, a):
         draw :class:`matplotlib.artist.Artist` instance *a* only --
         this is available only after the figure is drawn
         """
-        assert self._cachedRenderer is not None
+        if self._cachedRenderer is None:
+            msg = ('draw_artist can only be used after an initial draw which'
+                   ' caches the render')
+            raise AttributeError(msg)
         a.draw(self._cachedRenderer)
 
     def get_axes(self):
diff --git a/lib/matplotlib/finance.py b/lib/matplotlib/finance.py
index 02b020b58e08..630a531dc885 100644
--- a/lib/matplotlib/finance.py
+++ b/lib/matplotlib/finance.py
@@ -736,7 +736,7 @@ def candlestick_ohlc(ax, quotes, width=0.2, colorup='k', colordown='r',
 
 
 def _candlestick(ax, quotes, width=0.2, colorup='k', colordown='r',
-                alpha=1.0, ochl=True):
+                 alpha=1.0, ochl=True):
 
     """
     Plot the time, open, high, low, close as a vertical line ranging
@@ -816,6 +816,63 @@ def _candlestick(ax, quotes, width=0.2, colorup='k', colordown='r',
     return lines, patches
 
 
+def _check_input(opens, closes, highs, lows, miss=-1):
+    """Checks that *opens*, *highs*, *lows* and *closes* have the same length.
+    NOTE: this code assumes if any value open, high, low, close is
+    missing (*-1*) they all are missing
+
+    Parameters
+    ----------
+    ax : `Axes`
+        an Axes instance to plot to
+    opens : sequence
+        sequence of opening values
+    highs : sequence
+        sequence of high values
+    lows : sequence
+        sequence of low values
+    closes : sequence
+        sequence of closing values
+    miss : int
+        identifier of the missing data
+
+    Raises
+    ------
+    ValueError
+        if the input sequences don't have the same length
+    """
+
+    def _missing(sequence, miss=-1):
+        """Returns the index in *sequence* of the missing data, identified by
+        *miss*
+
+        Parameters
+        ----------
+        sequence :
+            sequence to evaluate
+        miss :
+            identifier of the missing data
+
+        Returns
+        -------
+        where_miss: numpy.ndarray
+            indices of the missing data
+        """
+        return np.where(np.array(sequence) == miss)[0]
+
+    same_length = len(opens) == len(highs) == len(lows) == len(closes)
+    _missopens = _missing(opens)
+    same_missing = ((_missopens == _missing(highs)).all() and
+                    (_missopens == _missing(lows)).all() and
+                    (_missopens == _missing(closes)).all())
+
+    if not (same_length and same_missing):
+        msg = ("*opens*, *highs*, *lows* and *closes* must have the same"
+               " length. NOTE: this code assumes if any value open, high,"
+               " low, close is missing (*-1*) they all must be missing.")
+        raise ValueError(msg)
+
+
 def plot_day_summary2_ochl(ax, opens, closes, highs, lows, ticksize=4,
                           colorup='k', colordown='r',
                           ):
@@ -860,6 +917,9 @@ def plot_day_summary2_ohlc(ax, opens, highs, lows, closes, ticksize=4,
     """Represent the time, open, high, low, close as a vertical line
     ranging from low to high.  The left tick is the open and the right
     tick is the close.
+    *opens*, *highs*, *lows* and *closes* must have the same length.
+    NOTE: this code assumes if any value open, high, low, close is
+    missing (*-1*) they all are missing
 
     Parameters
     ----------
@@ -885,8 +945,8 @@ def plot_day_summary2_ohlc(ax, opens, highs, lows, closes, ticksize=4,
     ret : list
         a list of lines added to the axes
     """
-    # note this code assumes if any value open, high, low, close is
-    # missing they all are missing
+
+    _check_input(opens, highs, lows, closes)
 
     rangeSegments = [((i, low), (i, high)) for i, low, high in
                      zip(xrange(len(lows)), lows, highs) if low != -1]
@@ -919,10 +979,6 @@ def plot_day_summary2_ohlc(ax, opens, highs, lows, closes, ticksize=4,
     colors = [colord[open < close] for open, close in
               zip(opens, closes) if open != -1 and close != -1]
 
-    assert(len(rangeSegments) == len(offsetsOpen))
-    assert(len(offsetsOpen) == len(offsetsClose))
-    assert(len(offsetsClose) == len(colors))
-
     useAA = 0,   # use tuple here
     lw = 1,      # and here
     rangeCollection = LineCollection(rangeSegments,
@@ -1012,6 +1068,9 @@ def candlestick2_ohlc(ax, opens, highs, lows, closes, width=4,
     """Represent the open, close as a bar line and high low range as a
     vertical line.
 
+    NOTE: this code assumes if any value open, low, high, close is
+    missing they all are missing
+
 
     Parameters
     ----------
@@ -1040,8 +1099,7 @@ def candlestick2_ohlc(ax, opens, highs, lows, closes, width=4,
         (lineCollection, barCollection)
     """
 
-    # note this code assumes if any value open, low, high, close is
-    # missing they all are missing
+    _check_input(opens, highs, lows, closes)
 
     delta = width / 2.
     barVerts = [((i - delta, open),
@@ -1066,8 +1124,6 @@ def candlestick2_ohlc(ax, opens, highs, lows, closes, width=4,
               for open, close in zip(opens, closes)
               if open != -1 and close != -1]
 
-    assert(len(barVerts) == len(rangeSegments))
-
     useAA = 0,  # use tuple here
     lw = 0.5,   # and here
     rangeCollection = LineCollection(rangeSegments,
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
index 2428147218bb..a0f12fb946a3 100644
--- a/lib/matplotlib/image.py
+++ b/lib/matplotlib/image.py
@@ -520,7 +520,8 @@ def set_filterrad(self, filterrad):
         ACCEPTS: positive float
         """
         r = float(filterrad)
-        assert(r > 0)
+        if r <= 0:
+            raise ValueError("The filter radius must be a positive number")
         self._filterrad = r
 
     def get_filterrad(self):
diff --git a/lib/matplotlib/mlab.py b/lib/matplotlib/mlab.py
index 2baedbb952aa..75127fe55e83 100644
--- a/lib/matplotlib/mlab.py
+++ b/lib/matplotlib/mlab.py
@@ -1485,7 +1485,8 @@ def cohere_pairs(X, ij, NFFT=256, Fs=2, detrend=detrend_none,
     # of every channel.  If preferSpeedOverMemory, cache the conjugate
     # as well
     if cbook.iterable(window):
-        assert(len(window) == NFFT)
+        if len(window) != NFFT:
+            raise ValueError("The length of the window must be equal to NFFT")
         windowVals = window
     else:
         windowVals = window(np.ones(NFFT, X.dtype))
@@ -3577,7 +3578,8 @@ def stineman_interp(xi, x, y, yp=None):
     # Cast key variables as float.
     x = np.asarray(x, np.float_)
     y = np.asarray(y, np.float_)
-    assert x.shape == y.shape
+    if x.shape != y.shape:
+        raise ValueError("'x' and 'y' must be of same shape")
 
     if yp is None:
         yp = slopes(x, y)
@@ -3824,7 +3826,8 @@ def poly_below(xmin, xs, ys):
     ys = numpy.asarray(ys)
     Nx = len(xs)
     Ny = len(ys)
-    assert(Nx == Ny)
+    if Nx != Ny:
+        raise ValueError("'xs' and 'ys' must have the same length")
     x = xmin*numpy.ones(2*Nx)
     y = numpy.ones(2*Nx)
     x[:Nx] = xs
diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
index fa82e91607cc..eea54c41fa64 100644
--- a/lib/matplotlib/patches.py
+++ b/lib/matplotlib/patches.py
@@ -3094,10 +3094,10 @@ def ensure_quadratic_bezier(path):
             its control points if true.
             """
             segments = list(path.iter_segments())
-            assert len(segments) == 2
-
-            assert segments[0][1] == Path.MOVETO
-            assert segments[1][1] == Path.CURVE3
+            if ((len(segments) != 2) or (segments[0][1] != Path.MOVETO) or
+                    (segments[1][1] != Path.CURVE3)):
+                msg = "'path' it's not a valid quadratic bezier curve"
+                raise ValueError(msg)
 
             return list(segments[0][0]) + list(segments[1][0])
 
@@ -3805,7 +3805,7 @@ def transmute(self, path, mutation_size, linewidth):
 
     class Wedge(_Base):
         """
-        Wedge(?) shape. Only wokrs with a quadratic bezier curve.  The
+        Wedge(?) shape. Only works with a quadratic bezier curve.  The
         begin point has a width of the tail_width and the end point has a
         width of 0. At the middle, the width is shrink_factor*tail_width.
 
diff --git a/lib/matplotlib/path.py b/lib/matplotlib/path.py
index 90acebb999ae..6f9af4296af5 100644
--- a/lib/matplotlib/path.py
+++ b/lib/matplotlib/path.py
@@ -136,21 +136,26 @@ def __init__(self, vertices, codes=None, _interpolation_steps=1,
         else:
             vertices = np.asarray(vertices, np.float_)
 
+        if (vertices.ndim != 2) or (vertices.shape[1] != 2):
+            msg = "'vertices' must be a 2D list or array with shape Nx2"
+            raise ValueError(msg)
+
         if codes is not None:
             codes = np.asarray(codes, self.code_type)
-            assert codes.ndim == 1
-            assert len(codes) == len(vertices)
-            if len(codes):
-                assert codes[0] == self.MOVETO
+            if (codes.ndim != 1) or len(codes) != len(vertices):
+                msg = ("'codes' must be a 1D list or array with the same"
+                       " length of 'vertices'")
+                raise ValueError(msg)
+            if len(codes) and codes[0] != self.MOVETO:
+                msg = ("The first element of 'code' must be equal to 'MOVETO':"
+                       " {0}")
+                raise ValueError(msg.format(self.MOVETO))
         elif closed:
             codes = np.empty(len(vertices), dtype=self.code_type)
             codes[0] = self.MOVETO
             codes[1:-1] = self.LINETO
             codes[-1] = self.CLOSEPOLY
 
-        assert vertices.ndim == 2
-        assert vertices.shape[1] == 2
-
         self._vertices = vertices
         self._codes = codes
         self._interpolation_steps = _interpolation_steps
@@ -320,7 +325,8 @@ def make_compound_path_from_polys(cls, XY):
         # the CLOSEPOLY; the vert for the closepoly is ignored but we still
         # need it to keep the codes aligned with the vertices
         numpolys, numsides, two = XY.shape
-        assert(two == 2)
+        if two != 2:
+            raise ValueError("The third dimension of 'XY' must be 2")
         stride = numsides + 1
         nverts = numpolys * stride
         verts = np.zeros((nverts, 2))
diff --git a/lib/matplotlib/projections/geo.py b/lib/matplotlib/projections/geo.py
index a81dad29782c..827ed86c2d21 100644
--- a/lib/matplotlib/projections/geo.py
+++ b/lib/matplotlib/projections/geo.py
@@ -129,7 +129,9 @@ def _get_affine_transform(self):
             .translate(0.5, 0.5)
 
     def get_xaxis_transform(self,which='grid'):
-        assert which in ['tick1','tick2','grid']
+        if which not in ['tick1','tick2','grid']:
+            msg = "'which' must be on of [ 'tick1' | 'tick2' | 'grid' ]"
+            raise ValueError(msg)
         return self._xaxis_transform
 
     def get_xaxis_text1_transform(self, pad):
@@ -139,7 +141,9 @@ def get_xaxis_text2_transform(self, pad):
         return self._xaxis_text2_transform, 'top', 'center'
 
     def get_yaxis_transform(self,which='grid'):
-        assert which in ['tick1','tick2','grid']
+        if which not in ['tick1','tick2','grid']:
+            msg = "'which' must be one of [ 'tick1' | 'tick2' | 'grid' ]"
+            raise ValueError(msg)
         return self._yaxis_transform
 
     def get_yaxis_text1_transform(self, pad):
diff --git a/lib/matplotlib/projections/polar.py b/lib/matplotlib/projections/polar.py
index 5e815a0654a6..c78faf7f728a 100644
--- a/lib/matplotlib/projections/polar.py
+++ b/lib/matplotlib/projections/polar.py
@@ -340,7 +340,9 @@ def _set_lim_and_transforms(self):
             )
 
     def get_xaxis_transform(self,which='grid'):
-        assert which in ['tick1','tick2','grid']
+        if which not in ['tick1','tick2','grid']:
+            msg = "'which' must be one of [ 'tick1' | 'tick2' | 'grid' ]"
+            raise ValueError(msg)
         return self._xaxis_transform
 
     def get_xaxis_text1_transform(self, pad):
@@ -350,7 +352,9 @@ def get_xaxis_text2_transform(self, pad):
         return self._xaxis_text2_transform, 'center', 'center'
 
     def get_yaxis_transform(self,which='grid'):
-        assert which in ['tick1','tick2','grid']
+        if which not in ['tick1','tick2','grid']:
+            msg = "'which' must be on of [ 'tick1' | 'tick2' | 'grid' ]"
+            raise ValueError(msg)
         return self._yaxis_transform
 
     def get_yaxis_text1_transform(self, pad):
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
index 51289832c6b1..38e8f705e486 100644
--- a/lib/matplotlib/rcsetup.py
+++ b/lib/matplotlib/rcsetup.py
@@ -272,19 +272,21 @@ def validate_colorlist(s):
     'return a list of colorspecs'
     if isinstance(s, six.string_types):
         return [validate_color(c.strip()) for c in s.split(',')]
-    else:
-        assert type(s) in [list, tuple]
+    elif type(s) in (list, tuple):
         return [validate_color(c) for c in s]
-
+    else:
+        msg = "'s' must be of type [ string | list | tuple ]"
+        raise ValueError(msg)
 
 def validate_stringlist(s):
     'return a list'
     if isinstance(s, six.string_types):
         return [six.text_type(v.strip()) for v in s.split(',') if v.strip()]
-    else:
-        assert type(s) in [list, tuple]
+    elif type(s) in (list, tuple):
         return [six.text_type(v) for v in s if v]
-
+    else:
+        msg = "'s' must be of type [ string | list | tuple ]"
+        raise ValueError(msg)
 
 validate_orientation = ValidateInStrings(
     'orientation', ['landscape', 'portrait'])
diff --git a/lib/matplotlib/sankey.py b/lib/matplotlib/sankey.py
index b5f8e735e066..c9af96fd9af2 100755
--- a/lib/matplotlib/sankey.py
+++ b/lib/matplotlib/sankey.py
@@ -29,7 +29,8 @@
 #     argument specifies the direction of the arrows.  The "main"
 #     inputs/outputs are now specified via an orientation of 0, and there may
 #     be several of each.
-#   --Added assertions to catch common calling errors
+#   --Changed assertions to ValueError to catch common calling errors (by
+#     Francesco Montesano, franz.bergesung@gmail.com)
 #   --Added the physical unit as a string argument to be used in the labels, so
 #     that the values of the flows can usually be applied automatically
 #   --Added an argument for a minimum magnitude below which flows are not shown
@@ -150,17 +151,21 @@ def __init__(self, ax=None, scale=1.0, unit='', format='%G', gap=0.25,
             .. plot:: mpl_examples/api/sankey_demo_basics.py
         """
         # Check the arguments.
-        assert gap >= 0, (
+        if gap < 0:
+            raise ValueError(
             "The gap is negative.\nThis isn't allowed because it "
             "would cause the paths to overlap.")
-        assert radius <= gap, (
+        if radius > gap:
+            raise ValueError(
             "The inner radius is greater than the path spacing.\n"
             "This isn't allowed because it would cause the paths to overlap.")
-        assert head_angle >= 0, (
+        if head_angle < 0:
+            raise ValueError(
             "The angle is negative.\nThis isn't allowed "
             "because it would cause inputs to look like "
             "outputs and vice versa.")
-        assert tolerance >= 0, (
+        if tolerance < 0:
+            raise ValueError(
             "The tolerance is negative.\nIt must be a magnitude.")
 
         # Create axes if necessary.
@@ -470,20 +475,23 @@ def add(self, patchlabel='', flows=None, orientations=None, labels='',
             rotation /= 90.0
         if orientations is None:
             orientations = [0, 0]
-        assert len(orientations) == n, (
+        if len(orientations) != n:
+            raise ValueError(
             "orientations and flows must have the same length.\n"
             "orientations has length %d, but flows has length %d."
             % (len(orientations), n))
         if labels != '' and getattr(labels, '__iter__', False):
             # iterable() isn't used because it would give True if labels is a
             # string
-            assert len(labels) == n, (
+            if len(labels) != n:
+                raise ValueError(
                 "If labels is a list, then labels and flows must have the "
                 "same length.\nlabels has length %d, but flows has length %d."
                 % (len(labels), n))
         else:
             labels = [labels] * n
-        assert trunklength >= 0, (
+        if trunklength < 0:
+            raise ValueError(
             "trunklength is negative.\nThis isn't allowed, because it would "
             "cause poor layout.")
         if np.absolute(np.sum(flows)) > self.tolerance:
@@ -504,28 +512,35 @@ def add(self, patchlabel='', flows=None, orientations=None, labels='',
                 "cause poor layout.\nConsider changing the scale so"
                 " that the scaled sum is approximately 1.0." % gain, 'helpful')
         if prior is not None:
-            assert prior >= 0, "The index of the prior diagram is negative."
-            assert min(connect) >= 0, (
+            if prior < 0:
+                raise ValueError("The index of the prior diagram is negative.")
+            if min(connect) < 0:
+                raise ValueError(
                 "At least one of the connection indices is negative.")
-            assert prior < len(self.diagrams), (
+            if prior >= len(self.diagrams):
+                raise ValueError(
                 "The index of the prior diagram is %d, but there are "
                 "only %d other diagrams.\nThe index is zero-based."
                 % (prior, len(self.diagrams)))
-            assert connect[0] < len(self.diagrams[prior].flows), (
+            if connect[0] >= len(self.diagrams[prior].flows):
+                raise ValueError(
                 "The connection index to the source diagram is %d, but "
                 "that diagram has only %d flows.\nThe index is zero-based."
                 % (connect[0], len(self.diagrams[prior].flows)))
-            assert connect[1] < n, (
+            if connect[1] >= n:
+                raise ValueError(
                 "The connection index to this diagram is %d, but this diagram"
                 "has only %d flows.\n The index is zero-based."
                 % (connect[1], n))
-            assert self.diagrams[prior].angles[connect[0]] is not None, (
+            if self.diagrams[prior].angles[connect[0]] is None:
+                raise ValueError(
                 "The connection cannot be made.  Check that the magnitude "
                 "of flow %d of diagram %d is greater than or equal to the "
                 "specified tolerance." % (connect[0], prior))
             flow_error = (self.diagrams[prior].flows[connect[0]] +
                           flows[connect[1]])
-            assert abs(flow_error) < self.tolerance, (
+            if abs(flow_error) >= self.tolerance:
+                raise ValueError(
                 "The scaled sum of the connected flows is %f, which is not "
                 "within the tolerance (%f)." % (flow_error, self.tolerance))
 
@@ -556,9 +571,10 @@ def add(self, patchlabel='', flows=None, orientations=None, labels='',
                 if is_input is not None:
                     angles[i] = RIGHT
             else:
-                assert orient == -1, (
+                if orient != -1:
+                    raise ValueError(
                     "The value of orientations[%d] is %d, "
-                    "but it must be -1, 0, or 1." % (i, orient))
+                    "but it must be [ -1 | 0 | 1 ]." % (i, orient))
                 if is_input:
                     angles[i] = UP
                 elif not is_input:
@@ -566,7 +582,8 @@ def add(self, patchlabel='', flows=None, orientations=None, labels='',
 
         # Justify the lengths of the paths.
         if iterable(pathlengths):
-            assert len(pathlengths) == n, (
+            if len(pathlengths) != n:
+                raise ValueError(
                 "If pathlengths is a list, then pathlengths and flows must "
                 "have the same length.\npathlengths has length %d, but flows "
                 "has length %d." % (len(pathlengths), n))
diff --git a/lib/matplotlib/scale.py b/lib/matplotlib/scale.py
index 69158ad8b23f..7193d1e7edf6 100644
--- a/lib/matplotlib/scale.py
+++ b/lib/matplotlib/scale.py
@@ -440,9 +440,12 @@ def __init__(self, axis, **kwargs):
             subs = kwargs.pop('subsy', None)
             linscale = kwargs.pop('linscaley', 1.0)
 
-        assert base > 1.0
-        assert linthresh > 0.0
-        assert linscale > 0.0
+        if base <= 1.0:
+            raise ValueError("'basex/basey' must be larger than 1")
+        if linthresh <= 0.0:
+            raise ValueError("'linthreshx/linthreshy' must be positive")
+        if linscale <= 0.0:
+            raise ValueError("'linscalex/linthreshy' must be positive")
 
         self._transform = self.SymmetricalLogTransform(base,
                                                        linthresh,
diff --git a/lib/matplotlib/spines.py b/lib/matplotlib/spines.py
index 2673c46b92f2..f73ee15fbc73 100644
--- a/lib/matplotlib/spines.py
+++ b/lib/matplotlib/spines.py
@@ -71,7 +71,9 @@ def __init__(self, axes, spine_type, path, **kwargs):
         # non-rectangular axes is currently implemented, and this lets
         # them pass through the spines machinery without errors.)
         self._position = None
-        assert isinstance(path, matplotlib.path.Path)
+        if not isinstance(path, matplotlib.path.Path):
+            msg = "'path' must be an instance of 'matplotlib.path.Path'"
+            raise ValueError(msg)
         self._path = path
 
         # To support drawing both linear and circular spines, this
@@ -176,7 +178,8 @@ def is_frame_like(self):
                 position = ('axes', 0.5)
             elif position == 'zero':
                 position = ('data', 0)
-        assert len(position) == 2, "position should be 2-tuple"
+        if len(position) != 2:
+            raise ValueError("position should be 2-tuple")
         position_type, amount = position
         if position_type == 'outward' and amount == 0:
             return True
@@ -372,8 +375,12 @@ def set_position(self, position):
             # special positions
             pass
         else:
-            assert len(position) == 2, "position should be 'center' or 2-tuple"
-            assert position[0] in ['outward', 'axes', 'data']
+            if len(position) != 2:
+                raise ValueError("position should be 'center' or 2-tuple")
+            if position[0] not in ['outward', 'axes', 'data']:
+                msg = ("position[0] should be in [ 'outward' | 'axes' |"
+                       " 'data' ]")
+                raise ValueError(msg)
         self._position = position
         self._calc_offset_transform()
 
diff --git a/lib/matplotlib/streamplot.py b/lib/matplotlib/streamplot.py
index 1bfacdb70339..3708afa647db 100644
--- a/lib/matplotlib/streamplot.py
+++ b/lib/matplotlib/streamplot.py
@@ -90,7 +90,9 @@ def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
 
     use_multicolor_lines = isinstance(color, np.ndarray)
     if use_multicolor_lines:
-        assert color.shape == grid.shape
+        if color.shape != grid.shape:
+            msg = "If 'color' is given, must have the shape of 'Grid(x,y)'"
+            raise ValueError(msg)
         line_colors = []
         color = np.ma.masked_invalid(color)
     else:
@@ -98,7 +100,9 @@ def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
         arrow_kw['color'] = color
 
     if isinstance(linewidth, np.ndarray):
-        assert linewidth.shape == grid.shape
+        if linewidth.shape != grid.shape:
+            msg = "If 'linewidth' is given, must have the shape of 'Grid(x,y)'"
+            raise ValueError(msg)
         line_kw['linewidth'] = []
     else:
         line_kw['linewidth'] = linewidth
@@ -108,8 +112,9 @@ def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
     arrow_kw['zorder'] = zorder
 
     ## Sanity checks.
-    assert u.shape == grid.shape
-    assert v.shape == grid.shape
+    if (u.shape != grid.shape) or (v.shape != grid.shape):
+        msg = "'u' and 'v' must be of shape 'Grid(x,y)'"
+        raise ValueError(msg)
 
     u = np.ma.masked_invalid(u)
     v = np.ma.masked_invalid(v)
@@ -256,19 +261,25 @@ class Grid(object):
     """Grid of data."""
     def __init__(self, x, y):
 
-        if len(x.shape) == 2:
-            x_row = x[0]
-            assert np.allclose(x_row, x)
+        if x.ndim == 1:
+            pass
+        elif x.ndim == 2:
+            x_row = x[0, :]
+            if not np.allclose(x_row, x):
+                raise ValueError("The rows of 'x' must be equal")
             x = x_row
         else:
-            assert len(x.shape) == 1
+            raise ValueError("'x' can have at maximum 2 dimensions")
 
-        if len(y.shape) == 2:
+        if y.ndim == 1:
+            pass
+        elif y.ndim == 2:
             y_col = y[:, 0]
-            assert np.allclose(y_col, y.T)
+            if not np.allclose(y_col, y.T):
+                raise ValueError("The columns of 'y' must be equal")
             y = y_col
         else:
-            assert len(y.shape) == 1
+            raise ValueError("'y' can have at maximum 2 dimensions")
 
         self.nx = len(x)
         self.ny = len(y)
@@ -304,10 +315,12 @@ class StreamMask(object):
 
     def __init__(self, density):
         if np.isscalar(density):
-            assert density > 0
+            if density <= 0:
+                raise ValueError("If a scalar, 'density' must be positive")
             self.nx = self.ny = int(30 * density)
         else:
-            assert len(density) == 2
+            if len(density) != 2:
+                raise ValueError("'density' can have at maximum 2 dimensions")
             self.nx = int(30 * density[0])
             self.ny = int(30 * density[1])
         self._mask = np.zeros((self.ny, self.nx))
diff --git a/lib/matplotlib/table.py b/lib/matplotlib/table.py
index 497c1eb49ad4..c918457d3479 100644
--- a/lib/matplotlib/table.py
+++ b/lib/matplotlib/table.py
@@ -154,7 +154,7 @@ class Table(Artist):
 
     Each entry in the table can be either text or patches.
 
-    Column widths and row heights for the table can be specifified.
+    Column widths and row heights for the table can be specified.
 
     Return value is a sequence of text, line and patch instances that make
     up the table
@@ -482,12 +482,17 @@ def table(ax,
     rows = len(cellText)
     cols = len(cellText[0])
     for row in cellText:
-        assert len(row) == cols
+        if len(row) != cols:
+            msg = "Each row in 'cellText' must have {0} columns"
+            raise ValueError(msg.format(cols))
 
     if cellColours is not None:
-        assert len(cellColours) == rows
+        if len(cellColours) != rows:
+            raise ValueError("'cellColours' must have {0} rows".format(rows))
         for row in cellColours:
-            assert len(row) == cols
+            if len(row) != cols:
+                msg = "Each row in 'cellColours' must have {0} columns"
+                raise ValueError(msg.format(cols))
     else:
         cellColours = ['w' * cols] * rows
 
@@ -506,7 +511,8 @@ def table(ax,
         rowColours = 'w' * rows
 
     if rowLabels is not None:
-        assert len(rowLabels) == rows
+        if len(rowLabels) != rows:
+            raise ValueError("'rowLabels' must be of length {0}".format(rows))
 
     # If we have column labels, need to shift
     # the text and colour arrays down 1 row
@@ -519,9 +525,6 @@ def table(ax,
     elif colColours is None:
         colColours = 'w' * cols
 
-    if rowLabels is not None:
-        assert len(rowLabels) == rows
-
     # Set up cell colours if not given
     if cellColours is None:
         cellColours = ['w' * cols] * rows
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
index ec92da9ee246..8265cb77dad2 100644
--- a/lib/matplotlib/tests/test_rcparams.py
+++ b/lib/matplotlib/tests/test_rcparams.py
@@ -227,8 +227,8 @@ def test_validators():
                      (['a', 'b'], ['a', 'b']),
                      (('a', 'b'), ['a', 'b']),
                      ((1, 2), ['1', '2'])),
-            'fail': ((dict(), AssertionError),
-                     (1, AssertionError),)
+            'fail': ((dict(), ValueError),
+                     (1, ValueError),)
             },
         {'validator': validate_nseq_int(2),
          'success': ((_, [1, 2])
diff --git a/lib/matplotlib/text.py b/lib/matplotlib/text.py
index 14fdd4533bc3..034277fcd094 100644
--- a/lib/matplotlib/text.py
+++ b/lib/matplotlib/text.py
@@ -1474,7 +1474,8 @@ def __init__(self, artist, ref_coord, unit="points"):
         self.set_unit(unit)
 
     def set_unit(self, unit):
-        assert unit in ["points", "pixels"]
+        if unit not in ["points", "pixels"]:
+            raise ValueError("'unit' must be one of [ 'points' | 'pixels' ]")
         self._unit = unit
 
     def get_unit(self):
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index b374d5ce5693..69075fd9595a 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -464,7 +464,8 @@ def set_powerlimits(self, lims):
         greater than 1e4.
         See also :meth:`set_scientific`.
         '''
-        assert len(lims) == 2, "argument must be a sequence of length 2"
+        if len(lims) != 2:
+            raise ValueError("'lims' must be a sequence of length 2")
         self._powerlimits = lims
 
     def format_data_short(self, value):
@@ -1201,7 +1202,8 @@ def closeto(x, y):
 class Base(object):
     'this solution has some hacks to deal with floating point inaccuracies'
     def __init__(self, base):
-        assert(base > 0)
+        if base <= 0:
+            raise ValueError("'base' must be positive")
         self._base = base
 
     def lt(self, x):
diff --git a/lib/matplotlib/transforms.py b/lib/matplotlib/transforms.py
index e6c1c98ccfaf..fcac999115bf 100644
--- a/lib/matplotlib/transforms.py
+++ b/lib/matplotlib/transforms.py
@@ -593,7 +593,8 @@ def shrunk_to_aspect(self, box_aspect, container=None, fig_aspect=1.0):
         *fig_aspect*, so that *box_aspect* can also be given as a
         ratio of the absolute dimensions, not the relative dimensions.
         """
-        assert box_aspect > 0 and fig_aspect > 0
+        if box_aspect <= 0 or fig_aspect <= 0:
+            raise ValueError("'box_aspect' and 'fig_aspect' must be positive")
         if container is None:
             container = self
         w, h = container.size
@@ -719,7 +720,8 @@ def union(bboxes):
         """
         Return a :class:`Bbox` that contains all of the given bboxes.
         """
-        assert(len(bboxes))
+        if not len(bboxes):
+            raise ValueError("'bboxes' cannot be empty")
 
         if len(bboxes) == 1:
             return bboxes[0]
@@ -1062,10 +1064,15 @@ def __init__(self, bbox, transform, **kwargs):
 
         *transform*: a 2D :class:`Transform`
         """
-        assert bbox.is_bbox
-        assert isinstance(transform, Transform)
-        assert transform.input_dims == 2
-        assert transform.output_dims == 2
+        if not bbox.is_bbox:
+            raise ValueError("'bbox' is not a bbox")
+        if not isinstance(transform, Transform):
+            msg = ("'transform' must be an instance of"
+                   " 'matplotlib.transform.Transform'")
+            raise ValueError(msg)
+        if transform.input_dims != 2 or transform.output_dims != 2:
+            msg = "The input and output dimensions of 'transform' must be 2"
+            raise ValueError(msg)
 
         BboxBase.__init__(self, **kwargs)
         self._bbox = bbox
@@ -1384,7 +1391,9 @@ def transform_point(self, point):
         The transformed point is returned as a sequence of length
         :attr:`output_dims`.
         """
-        assert len(point) == self.input_dims
+        if len(point) != self.input_dims:
+            msg = "The length of 'point' must be 'self.input_dims'"
+            raise ValueError(msg)
         return self.transform(np.asarray([point]))[0]
 
     def transform_path(self, path):
@@ -1454,12 +1463,12 @@ def transform_angles(self, angles, pts, radians=False, pushoff=1e-5):
         if self.input_dims != 2 or self.output_dims != 2:
             raise NotImplementedError('Only defined in 2D')
 
-        # pts must be array with 2 columns for x,y
-        assert pts.shape[1] == 2
+        if pts.shape[1] != 2:
+            raise ValueError("'pts' must be array with 2 columns for x,y")
 
-        # angles must be a column vector and have same number of
-        # rows as pts
-        assert np.prod(angles.shape) == angles.shape[0] == pts.shape[0]
+        if angles.ndim!=1 or angles.shape[0] != pts.shape[0]:
+            msg = "'angles' must be a column vector and have same number of"
+            msg += " rows as 'pts'"
 
         # Convert to radians if desired
         if not radians:
@@ -1516,7 +1525,10 @@ def __init__(self, child):
         *child*: A class:`Transform` instance.  This child may later
         be replaced with :meth:`set`.
         """
-        assert isinstance(child, Transform)
+        if not isinstance(child, Transform):
+            msg = ("'child' must be an instance of"
+                   " 'matplotlib.transform.Transform'")
+            raise ValueError(msg)
         Transform.__init__(self)
         self.input_dims = child.input_dims
         self.output_dims = child.output_dims
@@ -1571,8 +1583,11 @@ def set(self, child):
         The new child must have the same number of input and output
         dimensions as the current child.
         """
-        assert child.input_dims == self.input_dims
-        assert child.output_dims == self.output_dims
+        if (child.input_dims != self.input_dims or
+                child.output_dims != self.output_dims):
+            msg = ("The new child must have the same number of input and"
+                   " output dimensions as the current child.")
+            raise ValueError(msg)
 
         self._set(child)
 
@@ -1822,7 +1837,10 @@ def set(self, other):
         Set this transformation from the frozen copy of another
         :class:`Affine2DBase` object.
         """
-        assert isinstance(other, Affine2DBase)
+        if not isinstance(other, Affine2DBase):
+            msg = ("'other' must be an instance of"
+                   " 'matplotlib.transform.Affine2DBase'")
+            raise ValueError(msg)
         self._mtx = other.get_matrix()
         self.invalidate()
 
@@ -2152,10 +2170,13 @@ def __init__(self, x_transform, y_transform, **kwargs):
         can determine automatically which kind of blended transform to
         create.
         """
-        assert x_transform.is_affine
-        assert y_transform.is_affine
-        assert x_transform.is_separable
-        assert y_transform.is_separable
+        is_affine = x_transform.is_affine and y_transform.is_affine
+        is_separable = x_transform.is_separable and y_transform.is_separable
+        is_correct = is_affine and is_separable
+        if not is_correct:
+            msg = ("Both *x_transform* and *y_transform* must be 2D affine"
+                   " transforms.")
+            raise ValueError(msg)
 
         Transform.__init__(self, **kwargs)
         self._x = x_transform
@@ -2232,7 +2253,10 @@ def __init__(self, a, b, **kwargs):
         which can automatically choose the best kind of composite
         transform instance to create.
         """
-        assert a.output_dims == b.input_dims
+        if a.output_dims != b.input_dims:
+            msg = ("The output dimension of 'a' must be equal to the input"
+                   " dimensions of 'b'")
+            raise ValueError(msg)
         self.input_dims = a.input_dims
         self.output_dims = b.output_dims
 
@@ -2357,11 +2381,14 @@ def __init__(self, a, b, **kwargs):
         which can automatically choose the best kind of composite
         transform instance to create.
         """
-        assert a.output_dims == b.input_dims
+        if not a.is_affine or not b.is_affine:
+            raise ValueError("'a' and 'b' must be affine transforms")
+        if a.output_dims != b.input_dims:
+            msg = ("The output dimension of 'a' must be equal to the input"
+                   " dimensions of 'b'")
+            raise ValueError(msg)
         self.input_dims = a.input_dims
         self.output_dims = b.output_dims
-        assert a.is_affine
-        assert b.is_affine
 
         Affine2DBase.__init__(self, **kwargs)
         self._a = a
@@ -2436,8 +2463,9 @@ def __init__(self, boxin, boxout, **kwargs):
         Create a new :class:`BboxTransform` that linearly transforms
         points from *boxin* to *boxout*.
         """
-        assert boxin.is_bbox
-        assert boxout.is_bbox
+        if not boxin.is_bbox or not boxout.is_bbox:
+            msg = "'boxin' and 'boxout' must be bbox"
+            raise ValueError(msg)
 
         Affine2DBase.__init__(self, **kwargs)
         self._boxin = boxin
@@ -2480,7 +2508,8 @@ def __init__(self, boxout, **kwargs):
         Create a new :class:`BboxTransformTo` that linearly transforms
         points from the unit bounding box to *boxout*.
         """
-        assert boxout.is_bbox
+        if not boxout.is_bbox:
+            raise ValueError("'boxout' must be bbox")
 
         Affine2DBase.__init__(self, **kwargs)
         self._boxout = boxout
@@ -2538,7 +2567,8 @@ class BboxTransformFrom(Affine2DBase):
     is_separable = True
 
     def __init__(self, boxin, **kwargs):
-        assert boxin.is_bbox
+        if not boxin.is_bbox:
+            raise ValueError("'boxin' must be bbox")
 
         Affine2DBase.__init__(self, **kwargs)
         self._boxin = boxin
@@ -2613,7 +2643,10 @@ def __init__(self, path, transform):
         Create a new :class:`TransformedPath` from the given
         :class:`~matplotlib.path.Path` and :class:`Transform`.
         """
-        assert isinstance(transform, Transform)
+        if not isinstance(transform, Transform):
+            msg = ("'transform' must be an instance of"
+                   " 'matplotlib.transform.Transform'")
+            raise ValueError(msg)
         TransformNode.__init__(self)
 
         self._path = path
diff --git a/lib/matplotlib/tri/tripcolor.py b/lib/matplotlib/tri/tripcolor.py
index 449ec03a0ed5..7db662ed2257 100644
--- a/lib/matplotlib/tri/tripcolor.py
+++ b/lib/matplotlib/tri/tripcolor.py
@@ -138,7 +138,9 @@ def tripcolor(ax, *args, **kwargs):
     collection.set_alpha(alpha)
     collection.set_array(C)
     if norm is not None:
-        assert(isinstance(norm, Normalize))
+        if not isinstance(norm, Normalize):
+            msg = "'norm' must be an instance of 'Normalize'"
+            raise ValueError(msg)
     collection.set_cmap(cmap)
     collection.set_norm(norm)
     if vmin is not None or vmax is not None:
diff --git a/lib/matplotlib/widgets.py b/lib/matplotlib/widgets.py
index 6675903b6877..6eba3574f7e4 100644
--- a/lib/matplotlib/widgets.py
+++ b/lib/matplotlib/widgets.py
@@ -1306,7 +1306,9 @@ def __init__(self, ax, onselect, direction, minspan=None, useblit=False,
         if rectprops is None:
             rectprops = dict(facecolor='red', alpha=0.5)
 
-        assert direction in ['horizontal', 'vertical'], 'Must choose horizontal or vertical for direction'
+        if direction not in ['horizontal', 'vertical']:
+            msg = "direction must be in [ 'horizontal' | 'vertical' ]"
+            raise ValueError(msg)
         self.direction = direction
 
         self.rect = None
@@ -1558,7 +1560,9 @@ def __init__(self, ax, onselect, drawtype='box',
         self.minspanx = minspanx
         self.minspany = minspany
 
-        assert(spancoords in ('data', 'pixels'))
+        if spancoords not in ('data', 'pixels'):
+            msg = "'spancoords' must be one of [ 'data' | 'pixels' ]"
+            raise ValueError(msg)
 
         self.spancoords = spancoords
         self.drawtype = drawtype