Bug summary

In matplotlib.colors._has_alpha_channel, it fails to identify colors with alpha channels in two contexts.

1. It assumes that any color specified as a string doesn't have an alpha channel. This is outdated, because hex strings that contain alpha values are permitted.
2. It assumes that tuples indicating the color and the alpha don't have alpha channels. This is because it assumes that the length has to be 4 for a non-string with an alpha channel.

Code for reproduction

import matplotlib.colors as mcolors

color = '#0f0f0f80'
print(mcolors._has_alpha_channel(color))

color = ('r', 0.5)
print(mcolors._has_alpha_channel(color))

Actual outcome

False
False

Expected outcome

True
True

Additional information

I can imagine a few fixes, but the code for creating an RGBA array from a color is a bit opaque to me. Also, the method is currently only used in lib/matplotlib/axis.py for the Tick class and in lib/matplotlib/test_colors.py. So maybe it's not that important, but it could be more useful if more flexible and accurate.

Motivation:
The motivation for this bug issue is the context where the alpha value should be set to a default value if the alpha is not provided by the user. This context would benefit greatly from a method that accurately determines whether a color or a sequence of colors have alpha values.

Operating system

OS/X

Matplotlib Version

3.9.0.dev687+g4099a45ee5

Matplotlib Backend

MacOSX

Python version

3.10.12

Jupyter version

No response

Installation

None