Fix `len(T)` so it doesn't upcast `Literal[:int]` from to `int` if returned from `T.__len__() ` by jorenham · Pull Request #11830 · python/typeshed

jorenham · 2024-04-24T15:13:47Z

Previously len(T) always returned and int, even if T.__len__() -> Literal[42]. I.e. T.__len__() and len(T) did not invariably match.

This PR fixes this discrepancy, by making len() generic on the return type of __len__, i.e. as len[N: int]: (SupportsLen[N]) -> N.

JelleZijlstra · 2024-04-24T15:18:39Z

This is incorrect, len() always returns an int in CPython.

JelleZijlstra · 2024-04-24T15:19:17Z

>>> class X:
...     def __len__(self): return "x"
... 
>>> len(X())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object cannot be interpreted as an integer

JelleZijlstra · 2024-04-24T15:20:13Z

And similarly if you return a subclass of int:

>>> class X:
...     def __len__(self): return True
... 
>>> len(X())
1

jorenham · 2024-04-24T15:22:48Z

@JelleZijlstra I realize that, but Literal[42] is also an int, and len() is able to return that.

jorenham · 2024-04-24T15:27:18Z

Unfortunately, using a N = TypeVar("N", int, LiteralInt) isn't possible at the moment. But the current upcasting behaviour from Literal[:int] to int is also incorrect.

jorenham · 2024-04-24T15:29:23Z

>>> class X:
...     def __len__(self): return "x"
... 
>>> len(X())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object cannot be interpreted as an integer

With this change, this example would still be rejected.

JelleZijlstra · 2024-04-24T15:39:17Z

As you can see, this leads to lots of new obscure errors. I don't think it's worth it.

github-actions · 2024-04-24T15:42:12Z

Diff from mypy_primer, showing the effect of this PR on open source code:

vision (https://github.com/pytorch/vision)
+ torchvision/models/segmentation/lraspp.py:173: error: Argument 2 to "_lraspp_mobilenetv3" has incompatible type "int | None"; expected "int"  [arg-type]
+ torchvision/models/segmentation/fcn.py:168: error: Argument 2 to "_fcn_resnet" has incompatible type "int | None"; expected "int"  [arg-type]
+ torchvision/models/segmentation/fcn.py:227: error: Argument 2 to "_fcn_resnet" has incompatible type "int | None"; expected "int"  [arg-type]
+ torchvision/models/segmentation/deeplabv3.py:275: error: Argument 2 to "_deeplabv3_resnet" has incompatible type "int | None"; expected "int"  [arg-type]
+ torchvision/models/segmentation/deeplabv3.py:331: error: Argument 2 to "_deeplabv3_resnet" has incompatible type "int | None"; expected "int"  [arg-type]
+ torchvision/models/segmentation/deeplabv3.py:385: error: Argument 2 to "_deeplabv3_mobilenetv3" has incompatible type "int | None"; expected "int"  [arg-type]

aiortc (https://github.com/aiortc/aiortc)
+ src/aiortc/rtcsctptransport.py:1731: error: Argument 1 to "len" has incompatible type "bytes"; expected "_SupportsLen[bool]"  [arg-type]
+ src/aiortc/rtcsctptransport.py:1731: note: Following member(s) of "bytes" have conflicts:
+ src/aiortc/rtcsctptransport.py:1731: note:     Expected:
+ src/aiortc/rtcsctptransport.py:1731: note:         def __len__(self) -> bool
+ src/aiortc/rtcsctptransport.py:1731: note:     Got:
+ src/aiortc/rtcsctptransport.py:1731: note:         def __len__(self) -> int

pylox (https://github.com/sco1/pylox)
- pylox/builtins/py_builtins.py:149: error: Argument 1 to "len" has incompatible type "LoxInstance"; expected "Sized"  [arg-type]
+ pylox/builtins/py_builtins.py:149: error: Argument 1 to "len" has incompatible type "LoxInstance"; expected "_SupportsLen[Never]"  [arg-type]

aiohttp-devtools (https://github.com/aio-libs/aiohttp-devtools)
+ aiohttp_devtools/runserver/serve.py:215: error: Invalid index type "AppKey[set[tuple[WebSocketResponse, str]]]" for "Application"; expected type "AppKey[_SupportsLen[int]]"  [index]
+ aiohttp_devtools/runserver/serve.py:245: error: Invalid index type "AppKey[set[tuple[WebSocketResponse, str]]]" for "Application"; expected type "AppKey[_SupportsLen[int]]"  [index]
+ aiohttp_devtools/runserver/serve.py:254: error: Invalid index type "AppKey[set[tuple[WebSocketResponse, str]]]" for "Application"; expected type "AppKey[_SupportsLen[int]]"  [index]
+ aiohttp_devtools/runserver/watch.py:80: error: Invalid index type "AppKey[set[tuple[WebSocketResponse, str]]]" for "Application"; expected type "AppKey[_SupportsLen[int]]"  [index]
+ aiohttp_devtools/runserver/watch.py:86: error: Invalid index type "AppKey[set[tuple[WebSocketResponse, str]]]" for "Application"; expected type "AppKey[_SupportsLen[int]]"  [index]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/datetimes.py:523: error: Argument 1 to "len" has incompatible type "ndarray[Any, dtype[signedinteger[Any]]]"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/arrays/datetimes.py:525: error: Argument 1 to "len" has incompatible type "ndarray[Any, dtype[signedinteger[Any]]]"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/io/json/_json.py:1237: error: Argument 1 to "len" has incompatible type "Series"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/io/json/_json.py:1237: note: Following member(s) of "Series" have conflicts:
+ pandas/io/json/_json.py:1237: note:     Expected:
+ pandas/io/json/_json.py:1237: note:         def __len__(self) -> bool
+ pandas/io/json/_json.py:1237: note:     Got:
+ pandas/io/json/_json.py:1237: note:         def __len__(self) -> int
+ pandas/io/formats/format.py:784: error: Argument 1 to "len" has incompatible type "list[Any]"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/frame.py:12174: error: Argument 1 to "len" has incompatible type "Index"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/frame.py:12174: note: Following member(s) of "Index" have conflicts:
+ pandas/core/frame.py:12174: note:     Expected:
+ pandas/core/frame.py:12174: note:         def __len__(self) -> bool
+ pandas/core/frame.py:12174: note:     Got:
+ pandas/core/frame.py:12174: note:         def __len__(self) -> int
+ pandas/core/frame.py:12211: error: Argument 1 to "len" has incompatible type "Index"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/frame.py:12211: note: Following member(s) of "Index" have conflicts:
+ pandas/core/frame.py:12211: note:     Expected:
+ pandas/core/frame.py:12211: note:         def __len__(self) -> bool
+ pandas/core/frame.py:12211: note:     Got:
+ pandas/core/frame.py:12211: note:         def __len__(self) -> int
+ pandas/core/reshape/pivot.py:172: error: Argument 1 to "len" has incompatible type "Index"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/reshape/pivot.py:172: note: Following member(s) of "Index" have conflicts:
+ pandas/core/reshape/pivot.py:172: note:     Expected:
+ pandas/core/reshape/pivot.py:172: note:         def __len__(self) -> bool
+ pandas/core/reshape/pivot.py:172: note:     Got:
+ pandas/core/reshape/pivot.py:172: note:         def __len__(self) -> int
+ pandas/core/reshape/merge.py:1366: error: Argument 1 to "len" has incompatible type "ExtensionArray | ndarray[Any, Any]"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/indexes/base.py:3682: error: Argument 1 to "len" has incompatible type "Index"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/indexes/base.py:3682: note: Following member(s) of "Index" have conflicts:
+ pandas/core/indexes/base.py:3682: note:     Expected:
+ pandas/core/indexes/base.py:3682: note:         def __len__(self) -> bool
+ pandas/core/indexes/base.py:3682: note:     Got:
+ pandas/core/indexes/base.py:3682: note:         def __len__(self) -> int
+ pandas/core/indexes/base.py:3913: error: Argument 1 to "len" has incompatible type "ndarray[Any, Any]"; expected "_SupportsLen[bool]"  [arg-type]
+ pandas/core/groupby/grouper.py:879: error: Argument 1 to "len" has incompatible type "NDFrameT"; expected "_SupportsLen[bool]"  [arg-type]

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/tools/web/app.py:554: error: Unsupported operand types for >= ("int" and "None")  [operator]
+ mitmproxy/tools/web/app.py:554: note: Left operand is of type "int | None"

pylint (https://github.com/pycqa/pylint)
+ pylint/lint/run.py:96: error: Returning Any from function declared to return "int"  [no-any-return]
+ pylint/lint/run.py:97: error: Returning Any from function declared to return "int"  [no-any-return]

hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
- src/hydra_zen/structured_configs/_implementations.py:2882: error: Argument 1 to "len" has incompatible type "str | int"; expected "Sized"  [arg-type]
+ src/hydra_zen/structured_configs/_implementations.py:2882: error: Argument 1 to "len" has incompatible type "str | int"; expected "_SupportsLen[int]"  [arg-type]
- src/hydra_zen/structured_configs/_implementations.py:2885: error: Argument 1 to "len" has incompatible type "str | int"; expected "Sized"  [arg-type]
+ src/hydra_zen/structured_configs/_implementations.py:2885: error: Argument 1 to "len" has incompatible type "str | int"; expected "_SupportsLen[int]"  [arg-type]

optuna (https://github.com/optuna/optuna)
+ optuna/study/_multi_objective.py:213: error: Unsupported operand types for < ("int" and "None")  [operator]
+ optuna/study/_multi_objective.py:213: note: Right operand is of type "int | None"
+ optuna/visualization/matplotlib/_pareto_front.py:195: error: Argument 1 to "len" has incompatible type "list[tuple[FrozenTrial, list[float]]]"; expected "_SupportsLen[bool]"  [arg-type]

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/python.py:1364: error: Need type annotation for "num_ids"  [var-annotated]
+ src/_pytest/python.py:1364: note: Error code "var-annotated" not covered by "type: ignore" comment

spark (https://github.com/apache/spark)
+ python/pyspark/core/context.py:786: error: Need type annotation for "size"  [var-annotated]
+ python/pyspark/pandas/series.py:4683: error: Need type annotation for "length"  [var-annotated]
+ python/pyspark/pandas/series.py:5462: error: Need type annotation for "length"  [var-annotated]

bandersnatch (https://github.com/pypa/bandersnatch)
+ src/bandersnatch_filter_plugins/allowlist_name.py: note: In member "pinned_version_exists" of class "AllowListRelease":
+ src/bandersnatch_filter_plugins/allowlist_name.py:210: error: Returning Any from function declared to return "bool"  [no-any-return]

ibis (https://github.com/ibis-project/ibis)
- ibis/formats/__init__.py:243: error: Argument 1 to "len" has incompatible type "T"; expected "Sized"  [arg-type]
+ ibis/formats/__init__.py:243: error: Argument 1 to "len" has incompatible type "T"; expected "_SupportsLen[Never]"  [arg-type]
- ibis/expr/types/pretty.py:239: error: Value of type variable "SupportsRichComparisonT" of "min" cannot be "int | None"  [type-var]
- ibis/expr/types/pretty.py:239: error: Incompatible types in assignment (expression has type "int | None", variable has type "int")  [assignment]
- ibis/expr/types/pretty.py:241: error: Incompatible types in assignment (expression has type "int | None", variable has type "int")  [assignment]
+ ibis/expr/api.py:312: error: Unsupported left operand type for != (Never)  [operator]
- ibis/expr/api.py:312: error: Argument 1 to "len" has incompatible type "Iterable[str] | None"; expected "Sized"  [arg-type]
+ ibis/expr/api.py:312: error: Argument 1 to "len" has incompatible type "Iterable[str] | None"; expected "_SupportsLen[Never]"  [arg-type]
- ibis/expr/api.py:312: error: Argument 1 to "len" has incompatible type "Iterable[str | DataType] | None"; expected "Sized"  [arg-type]
+ ibis/expr/api.py:312: error: Argument 1 to "len" has incompatible type "Iterable[str | DataType] | None"; expected "_SupportsLen[Never]"  [arg-type]
+ ibis/expr/api.py:505: error: Need type annotation for "provided_col"  [var-annotated]
- ibis/expr/api.py:505: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "Sized"  [arg-type]
+ ibis/expr/api.py:505: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "_SupportsLen[Never]"  [arg-type]
- ibis/backends/polars/__init__.py:321: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "Sized"  [arg-type]
+ ibis/backends/polars/__init__.py:321: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "_SupportsLen[Never]"  [arg-type]
- ibis/backends/polars/__init__.py:324: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "Sized"  [arg-type]
+ ibis/backends/polars/__init__.py:324: error: Argument 1 to "len" has incompatible type "Iterable[str]"; expected "_SupportsLen[Never]"  [arg-type]

black (https://github.com/psf/black)
+ src/blib2to3/pytree.py:945:9: error: Returning Any from function declared to return "bool"  [no-any-return]

jax (https://github.com/google/jax)
+ jax/_src/ad_checkpoint.py:624: error: Argument 1 to "tuple" has incompatible type "bool | Sequence[bool]"; expected "Iterable[bool]"  [arg-type]
+ jax/_src/interpreters/ad.py:685: error: Argument 1 to "tuple" has incompatible type "bool | Sequence[bool]"; expected "Iterable[bool]"  [arg-type]

streamlit (https://github.com/streamlit/streamlit)
+ lib/streamlit/runtime/secrets.py: note: In member "__len__" of class "AttrDict":
+ lib/streamlit/runtime/secrets.py:80:9: error: Returning Any from function declared to return "int"  [no-any-return]

jorenham · 2024-04-24T15:53:40Z

@JelleZijlstra Yea I tend to agree: It might be best to re-address this issue when something like a LiteralInt or Literal[int] type is introduced.

And for now, there's the simple workaround of using .__len__() instead of len().

jorenham and others added 3 commits April 24, 2024 16:55

test that len(T) and T.__len__() return the same type

e6f6b3b

prevent len(T) from upcasting literal ints returned from T.__len__()

899799e

[pre-commit.ci] auto fixes from pre-commit.com hooks

5096d68

workaround frozenset pyright error

1f4e827

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fix failing test on python 3.8

0ab5f90

jorenham closed this Apr 24, 2024

jorenham mentioned this pull request Dec 22, 2024

CI, TYP: Large-scale integration type-testing with mypy_primer numpy/numpy#28054

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Fix `len(T)` so it doesn't upcast `Literal[:int]` from to `int` if returned from `T.len()` - #11830

Fix `len(T)` so it doesn't upcast `Literal[:int]` from to `int` if returned from `T.len()` #11830
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jorenham:generic-len

jorenham commented Apr 24, 2024

Uh oh!

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jorenham commented Apr 24, 2024

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