10000 [Bug]: Type hints for ax argument in scale_factory does not allow None · Issue #29455 · matplotlib/matplotlib · GitHub
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[Bug]: Type hints for ax argument in scale_factory does not allow None #29455

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Illviljan opened this issue Jan 11, 2025 · 3 comments
Open

[Bug]: Type hints for ax argument in scale_factory does not allow None #29455

Illviljan opened this issue Jan 11, 2025 · 3 comments

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@Illviljan
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Illviljan commented Jan 11, 2025 •
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Bug summary

scale_factory requires Axis according to the type hints:

matplotlib/lib/matplotlib/scale.pyi

Line 177 in 8d64f03

def scale_factory(scale: str, axis: Axis, **kwargs) -> ScaleBase: ...

But ax is even discouraged in the code:

matplotlib/lib/matplotlib/scale.py

Lines 51 to 63 in 8d64f03

def __init__(self, axis):
r"""
Construct a new scale.
Notes
-----
The following note is for scale implementers.
For back-compatibility reasons, scales take an `~matplotlib.axis.Axis`
object as first argument. However, this argument should not
be used: a single scale object should be usable by multiple
`~matplotlib.axis.Axis`\es at the same time.
"""

matplotlib/lib/matplotlib/scale.pyi

Lines 8 to 9 in 8d64f03

class ScaleBase:
def __init__(self, axis: Axis | None) -> None: ...

Code for reproduction

from matplotlib.scale import scale_factory


scale_factory("linear", None)
# pyright:
# error: Argument of type "None" cannot be assigned to parameter "axis" of type "Axis" in function "scale_factory"
#   "None" is not assignable to "Axis" (reportArgumentType)

# mypy:
# error: Argument 2 to "scale_factory" has incompatible type "None"; expected "Axis"  [arg-type]

Actual outcome

# pyright:
# error: Argument of type "None" cannot be assigned to parameter "axis" of type "Axis" in function "scale_factory"
#   "None" is not assignable to "Axis" (reportArgumentType)

# mypy:
# error: Argument 2 to "scale_factory" has incompatible type "None"; expected "Axis"  [arg-type]

Expected outcome

mypy and pyright passing

Additional information

Using the same type hints as ScaleBase should solve this.

Operating system

No response

Matplotlib Version

3.9.2

Matplotlib Backend

No response

Python version

No response

Jupyter version

No response

Installation

None
@timhoffm
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timhoffm commented Jan 11, 2025 •
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Thanks for the report. The type hint is correct for now. An axis parameter must still be passed to all Scales by the caller for backwards compatibility. Maybe the note is not clear. The recommendation is that the scale does not make use of the parameter.

There’s a plan to remove the parameter, but that’ll take a while to make the transition smooth. See #29349.

@Illviljan
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Illviljan commented Jan 11, 2025 •
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I wasn't suggesting to remove the parameter, just to add None as well:

def scale_factory(scale: str, axis: Axis | None, **kwargs) -> ScaleBase: ...

@timhoffm
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timhoffm commented Jan 11, 2025 •
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While discouraged, and not used in Matplotlibs own scales anymore, 3rd party libs can still register scales that make use of this parameter. This would fail if somebody would pass None. It's a judgement call whether we allow None and let this error path slip through the type checker, or whether we still always require the Axis.

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