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Value of zero in array with dtype C char evaluates to 'True' #13973

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david-cortes opened this issue Jul 13, 2019 · 2 comments
Closed

Value of zero in array with dtype C char evaluates to 'True' #13973

david-cortes opened this issue Jul 13, 2019 · 2 comments

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@david-cortes
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david-cortes commented Jul 13, 2019 •
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Steps to reproduce:

  • Create an array of booleans represented as integers or some other type with something that can be evaluated to Python False.
  • Convert said array to C char.
  • Interpret any of those values as boolean.

Expected behavior: values corresponding to zero should be taken as False.

Actual behavior: all values are taken as True.

Example

import numpy as np, ctypes
arr = np.array([0, 0, 1])
arr_char = arr.astype(ctypes.c_char)
bool(arr_char[0])
>>>    True
@david-cortes david-cortes changed the title 'char' Array entries always evaluate to 'True' C 'char' Array entries always evaluate to 'True' Jul 13, 2019
@david-cortes david-cortes changed the title C 'char' Array entries always evaluate to 'True' Array entries of type C 'char' always evaluate to 'True' Jul 13, 2019
@david-cortes david-cortes changed the title Array entries of type C 'char' always evaluate to 'True' Value of zero in array with dtype C char evaluates to 'True' Jul 19, 2019
@WarrenWeckesser
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This is normal behavior.

Note that arr_char[0] is a numpy.bytes_ object with length 1:

In [36]: arr_char
Out[36]: array([b'0', b'0', b'1'], dtype='|S1')

In [37]: arr_char[0]
Out[37]: b'0'

The "truthiness" of a Python bytes object or a NumPy bytes_ object depends only on its length, not on the characters stored in the object; bytes objects with length 0 are False:

In [51]: bool(b'')
Out[51]: False

In [52]: bool(b'0')
Out[52]: True

In [53]: bool(np.bytes_(b''))
Out[53]: False

In [54]: bool(np.bytes_(b'0'))
Out[54]: True

@seberg
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seberg commented Nov 10, 2021

Agreed with Warren, just an addendum that while this one makes sense, there are a lot of corner cases that we badly need to align with Python strings: gh-9875

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@seberg @WarrenWeckesser @david-cortes