Emit redundant-expr warnings for if statements that are always true or always false.

[tyralla]

Fixes #19728

And thereby implements the two XXX comments of TypeChecker.visit_if_stmt.

The only surprise I encountered is that we must take care of while statements here, too, because while statements are transformed to if statements during type checking. It seems right to me to only emit warnings for while False but not for while True.

@A5rocks: Are you aware of other, uncovered special cases?

[tyralla]

Oh, 57 errors for Mypy itself, e.g. if TYPE_CHECKING. I will rethink this tomorrow...

[A5rocks]

We already kind of have a distinction between things that are true at compile time and run time for unreachability reporting, maybe that has a nice way to do that here? Probably not though.

(It's been too long for me to remember any special cases...)

[tyralla]

A few looks into the primer results (from bottom to top; without obvious repetitions):

nox

Mypy issues (in this case already under discussion in #11764).

x: str | None,
if callable(x):  # error: If condition is always false
    ...  # type: ignore[unreachable]

boostedblob

Additional runtime type checking.

def f(x: str | int) -> None:
    if not isinstance(x, str | int):  # error: If condition is always false
        raise ValueError

def g(x: str | int) -> None:
    if isinstance(x, str):
        return
    if isinstance(x, int):  # error: If condition is always true
       return
    raise AssertionError

scikit-build-core

Really redundant code (or at least one that needs explanations):

def f(env: dict[str, str] | None):
    if env:
        for key, value in env.items():
            if isinstance(value, bool):  # error: If condition is always false
                ...

import dataclasses

@dataclasses.dataclass
class Settings1:
    cmake: bool = False

@dataclasses.dataclass
class Settings2:
    cmake: bool = False

def f(s1: Settings1, s2: Settings2) -> None:
    if not s1.cmake or s2.cmake:
        ...
    if s2.cmake:  # error: If condition is always false
        ...

Based on these few examples, this PR seems correct. However, it somehow collides with exhaustiveness checking.

[tyralla]

I tried to filter out warnings for redundant if statements that obviously serve for dynamic type or exhaustiveness checking. The logic is very similar to the one implemented in TypeChecker.is_noop_for_reachability (there is currently even some code duplication). After revisiting some of the primer results, there seem to be two missed cases:

Assertion or raise statement not in else body:

def f(x: int | str) -> None:
    if isinstance(x, int):
        ...
    elif isinstance(x, str):
        ...
    assert False

Custom error handling method:

def f(x: int) -> None:
    if not isinstance(x, int):
        print("type error!")
        return None
    ....

Assertion or raise statement not in the first line of the if or else body:

def f(x: int) -> None:
    if not isinstance(x, int):
        msg = "type error!"
        raise TypeError(msg)
    ....

Missing assertion or raise statement:

def f(x: Literal[1, 2]) -> None:
    if x == 1:
        ...
    elif x == 2:
        ...
    return None

Return NotImplemented:

    def __add__(self, x: float) -> float:
        if not isinstance(name, float):
            return NotImplemented  # type: ignore[unreachable]
        ...

The current state of this PR also causes two unreachable warnings for reasons I have not yet investigated.

[A5rocks]

IMO this can check infer_condition_value (see unreachability.py) and see if it's MYPY_TRUE or MYPY_FALSE. I think that would be more resilient. It's a bit funny that that relies on a name, but I think it's better to match it anyways.

[tyralla]

Yes, you are probably right. At least, now things like if TYPE_CHECKING and TYPE_CHECKING: (and maybe more useful patterns) are possible.

The comparison with name appears less ideal than the original one with fullname to me, but I think this difference does not really matter. At least, it allowed me to add the test testNoRedundantExpressionWarningsForUsagesOfTYPECHECKING easily.

[A5rocks]

Yeah as you point out this seems to duplicate is_noop_for_reachability. Why not just call that? I think probably pass and ... as bodies are alright. (and any reasoning against them IMO also stands for disabling reachability warnings on them :^) )

[tyralla]

I often saw that someone used pass temporarily as the only statement of a if or else block and later added real code but forgot to remove the pass statement. For reachability analysis, this causes no problem:

from typing import Literal

x: Literal[1]

if x == 1:
    y = 1
else:
    pass
    y = 2  # error: Statement is unreachable

However, this PR only evaluates the block's first statement, which is maybe okay for raise but not for pass.

Maybe it is better to go through all lines and only silence the redundant-expr error if all of them are ignorable. In this case, yes, calling is_noop_for_reachability would be the right thing to do. I avoided this so far because of performance concerns. However, after rethinking, it seems unlikely that finding the first line that is not ignorable takes many tries.

[tyralla]

I did it so.

[A5rocks]

Hm. I would prefer if we could only exempt truthy while blocks if they are very trivial. Have you looked at only exempting int and bool literals? (or I guess, maybe all literals if we don't want to be too lintery...)

[tyralla]

No, I haven't, but your idea sounds reasonable.

[tyralla]

Done

[tyralla]

@A5rocks: Thanks for the remarks! I adjusted everything as suggested.

I think the only big open case left (for this PR) is:

def f(x: int | str) -> None:
    if isinstance(x, int):
        ...
    elif isinstance(x, str):
        ...
    assert False

I tend to make the block after the last if or elif statement available as a surrogate else block, but I am not sure if I can implement this cleanly. Or maybe we could check that the last if block returns None (in this case), but this seems too complicated/fragile. Alternatively, we could document that users must explicitly write the else block or use type: ignore if this is not what they want (possibly not a very good alternative):

def f(x: int | str) -> None:
    if isinstance(x, int):
        ...
    elif isinstance(x, str):
        ...
    else:
        assert False

[github-actions]

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

CPython (Argument Clinic) (https://github.com/python/cpython)
+ Tools/clinic/libclinic/clanguage.py:82: error: Statement is unreachable  [unreachable]

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/client_exceptions.py:323:4: error: If condition is always true  [redundant-expr]
+ aiohttp/client_exceptions.py:323:4: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-redundant-expr for more info
+ aiohttp/helpers.py:267:8: error: If condition is always false  [redundant-expr]
+ aiohttp/helpers.py:877:14: error: If condition is always true  [redundant-expr]
+ aiohttp/cookiejar.py:297:12: error: If condition is always false  [redundant-expr]
+ aiohttp/cookiejar.py:485:15: error: While condition is always false  [redundant-expr]
+ aiohttp/multipart.py:306:15: error: While condition is always true  [redundant-expr]
+ aiohttp/multipart.py:447:15: error: While condition is always true  [redundant-expr]
+ aiohttp/multipart.py:660:12: error: If condition is always false  [redundant-expr]
+ aiohttp/connector.py:159:12: error: If condition is always true  [redundant-expr]
+ aiohttp/connector.py:812:8: error: If condition is always false  [redundant-expr]
+ aiohttp/client_reqrep.py:169:4: error: If condition is always true  [redundant-expr]
+ aiohttp/client_reqrep.py:275:12: error: If condition is always true  [redundant-expr]
+ aiohttp/client.py:948:16: error: If condition is always true  [redundant-expr]
+ aiohttp/web_response.py:267:14: error: If condition is always true  [redundant-expr]
+ aiohttp/web_fileresponse.py:328:16: error: If condition is always true  [redundant-expr]
+ aiohttp/web_protocol.py:715:16: error: If condition is always false  [redundant-expr]
+ aiohttp/web_runner.py:263:20: error: If condition is always true  [redundant-expr]

mypy (https://github.com/python/mypy)
+ mypy/nodes.py:4533: error: If condition is always true  [redundant-expr]
+ mypy/nodes.py:4533: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-redundant-expr for more info
+ mypy/nodes.py:4582: error: If condition is always true  [redundant-expr]
+ mypy/nodes.py:4626: error: If condition is always true  [redundant-expr]
+ mypy/nodes.py:4831: error: If condition is always true  [redundant-expr]
+ mypy/traverser.py:126: error: If condition is always true  [redundant-expr]
+ mypy/traverser.py:195: error: If condition is always true  [redundant-expr]
+ mypy/traverser.py:201: error: If condition is always true  [redundant-expr]
+ mypy/server/astmerge.py:448: error: If condition is always true  [redundant-expr]
+ mypy/server/astmerge.py:460: error: If condition is always true  [redundant-expr]
+ mypy/server/astmerge.py:477: error: If condition is always true  [redundant-expr]
+ mypy/fixup.py:280: error: If condition is always true  [redundant-expr]
+ mypy/fixup.py:282: error: If condition is always true  [redundant-expr]
+ mypy/fixup.py:315: error: If condition is always true  [redundant-expr]
+ mypy/fixup.py:322: error: If condition is always true  [redundant-expr]
+ mypy/fixup.py:362: error: If condition is always true  [redundant-expr]
+ mypy/constraints.py:133: error: If condition is always false  [redundant-expr]
+ mypy/constraints.py:133: note: Error code "redundant-expr" not covered by "type: ignore" comment
+ mypy/messages.py:1254: error: If condition is always true  [redundant-expr]
+ mypy/fastparse.py:2077: error: If condition is always true  [redundant-expr]
+ mypyc/analysis/ircheck.py:307: error: If condition is always true  [redundant-expr]
+ mypy/server/astdiff.py:259: error: If condition is always true  [redundant-expr]
+ mypy/plugins/dataclasses.py:140: error: If condition is always true  [redundant-expr]
+ mypy/semanal.py:1003: error: If condition is always true  [redundant-expr]
+ mypy/semanal.py:1403: error: If condition is always true  [redundant-expr]
+ mypy/semanal.py:3872: error: If condition is always true  [redundant-expr]
+ mypy/semanal.py:5817: error: If condition is always true  [redundant-expr]
+ mypy/checkexpr.py:3546: error: Statement is unreachable  [unreachable]
+ mypy/checkexpr.py:3546: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-unreachable for more info
+ mypy/checker.py:2508: error: If condition is always true  [redundant-expr]
+ mypy/checker.py:8671: error: If condition is always true  [redundant-expr]
+ mypy/checker.py:8672: error: If condition is always true  [redundant-expr]
+ mypy/build.py:1138: error: If condition is always false  [redundant-expr]
+ mypy/build.py:1381: error: If condition is always false  [redundant-expr]
+ mypyc/irbuild/prepare.py:263: error: If condition is always true  [redundant-expr]
+ mypy/stubtest.py:349: error: If condition is always false  [redundant-expr]
+ mypy/stubtest.py:632: error: If condition is always false  [redundant-expr]
+ mypyc/irbuild/expression.py:919: error: If condition is always true  [redundant-expr]

pyproject-metadata (https://github.com/pypa/pyproject-metadata)
+ pyproject_metadata/pyproject.py:86: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:100: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:105: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:168: error: If condition is always true  [redundant-expr]
+ pyproject_metadata/pyproject.py:249: error: If condition is always true  [redundant-expr]
+ pyproject_metadata/pyproject.py:333: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:341: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:351: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:383: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:396: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/pyproject.py:408: error: If condition is always false  [redundant-expr]
+ pyproject_metadata/__init__.py:369: error: If condition is always false  [redundant-expr]

rich (https://github.com/Textualize/rich)
+ rich/text.py:990: error: If condition is always true  [redundant-expr]
+ rich/text.py:1241: error: If condition is always true  [redundant-expr]
+ rich/rule.py:96: error: If condition is always true  [redundant-expr]
+ rich/padding.py:71: error: If condition is always true  [redundant-expr]
+ rich/align.py:178: error: If condition is always true  [redundant-expr]

pyodide (https://github.com/pyodide/pyodide)
+ pyodide-build/pyodide_build/vendor/loky.py:166: error: If condition is always false  [redundant-expr]
+ pyodide-build/pyodide_build/common.py:268: error: If condition is always true  [redundant-expr]
+ pyodide-build/pyodide_build/recipe/skeleton.py:126: error: If condition is always true  [redundant-expr]
+ src/py/pyodide/http/pyxhr.py:188: error: If condition is always true  [redundant-expr]

packaging (https://github.com/pypa/packaging)
+ src/packaging/_manylinux.py:142: error: If condition is always false  [redundant-expr]
+ src/packaging/markers.py:344: error: If condition is always false  [redundant-expr]
+ src/packaging/metadata.py:386: error: If condition is always false  [redundant-expr]

poetry (https://github.com/python-poetry/poetry)
+ src/poetry/utils/_compat.py:39: error: If condition is always false  [redundant-expr]
+ src/poetry/pyproject/toml.py:48: error: If condition is always false  [redundant-expr]
+ src/poetry/utils/authenticator.py:164: error: If condition is always true  [redundant-expr]
+ src/poetry/utils/authenticator.py:263: error: If condition is always true  [redundant-expr]
+ src/poetry/config/source.py:23: error: If condition is always false  [redundant-expr]
+ src/poetry/utils/env/env_manager.py:196: error: If condition is always false  [redundant-expr]
+ src/poetry/utils/env/env_manager.py:377: error: If condition is always false  [redundant-expr]
+ src/poetry/installation/executor.py:819: error: If condition is always false  [redundant-expr]
+ src/poetry/console/commands/remove.py:70: error: If condition is always false  [redundant-expr]
+ tests/console/commands/test_install.py:155: error: If condition is always false  [redundant-expr]
+ tests/console/commands/test_build.py:184: error: If condition is always false  [redundant-expr]
+ src/poetry/console/commands/add.py:329: error: If condition is always true  [redundant-expr]

archinstall (https://github.com/archlinux/archinstall)
+ archinstall/lib/models/authentication.py:47: error: If condition is always false  [redundant-expr]
+ archinstall/lib/models/mirrors.py:100: error: If condition is always true  [redundant-expr]
+ archinstall/lib/installer.py:840: error: If condition is always true  [redundant-expr]
+ archinstall/lib/installer.py:946: error: If condition is always true  [redundant-expr]
+ archinstall/lib/authentication/authentication_menu.py:120: error: If condition is always true  [redundant-expr]
+ archinstall/lib/profile/profile_menu.py:142: error: If condition is always true  [redundant-expr]

build (https://github.com/pypa/build)
+ src/build/__main__.py:187: error: If condition is always true  [redundant-expr]

xarray (https://github.com/pydata/xarray)
+ xarray/compat/npcompat.py: note: In function "isdtype":
+ xarray/compat/npcompat.py:72: error: If condition is always false  [redundant-expr]
+ xarray/core/utils.py: note: In function "equivalent":
+ xarray/core/utils.py:266: error: If condition is always false  [redundant-expr]
+ xarray/backends/locks.py: note: In function "combine_locks":
+ xarray/backends/locks.py:267: error: If condition is always true  [redundant-expr]
+ xarray/core/indexing.py: note: In function "group_indexers_by_index":
+ xarray/core/indexing.py:147: error: If condition is always true  [redundant-expr]
+ xarray/core/indexing.py: note: In function "map_index_queries":
+ xarray/core/indexing.py:197: error: If condition is always false  [redundant-expr]
+ xarray/core/indexing.py: note: In function "_decompose_outer_indexer":
+ xarray/core/indexing.py:1398: error: If condition is always false  [redundant-expr]
+ xarray/core/indexing.py: note: At top level:
+ xarray/core/indexes.py: note: In member "from_variables" of class "PandasIndex":
+ xarray/core/indexes.py:722: error: If condition is always false  [redundant-expr]
+ xarray/core/indexes.py: note: In member "to_pandas_indexes" of class "Indexes":
+ xarray/core/indexes.py:1853: error: If condition is always false  [redundant-expr]
+ xarray/core/indexes.py:1853: error: If condition is always true  [redundant-expr]
+ xarray/core/indexes.py:1855: error: If condition is always true  [redundant-expr]
+ xarray/core/indexes.py: note: In member "copy_indexes" of class "Indexes":
+ xarray/core/indexes.py:1881: error: If condition is always false  [redundant-expr]
+ xarray/core/indexes.py:1881: error: If condition is always true  [redundant-expr]
+ xarray/plot/facetgrid.py: note: In member "set_axis_labels" of class "FacetGrid":
+ xarray/plot/facetgrid.py:852: error: If condition is always false  [redundant-expr]
+ xarray/plot/facetgrid.py: note: At top level:
+ xarray/core/variable.py: note: In member "_pad_options_dim_to_index" of class "Variable":
+ xarray/core/variable.py:1225: error: If condition is always false  [redundant-expr]
+ xarray/core/variable.py: note: At top level:
+ xarray/structure/merge.py: note: In function "coerce_pandas_values":
+ xarray/structure/merge.py:530: error: If condition is always false  [redundant-expr]
+ xarray/core/coordinates.py: note: In member "equals" of class "Coordinates":
+ xarray/core/coordinates.py:477: error: If condition is always false  [redundant-expr]
+ xarray/core/coordinates.py: note: In member "identical" of class "Coordinates":
+ xarray/core/coordinates.py:488: error: If condition is always false  [redundant-expr]
+ xarray/coding/cftimeindex.py: note: In member "shift" of class "CFTimeIndex":
+ xarray/coding/cftimeindex.py:508: error: If condition is always true  [redundant-expr]
+ xarray/conventions.py: note: In function "ensure_not_multiindex":
+ xarray/conventions.py:56: error: If condition is always false  [redundant-expr]
+ xarray/plot/dataarray_plot.py: note: In function "line":
+ xarray/plot/dataarray_plot.py:521: error: If condition is always true  [redundant-expr]
+ xarray/plot/dataarray_plot.py:524: error: If condition is always true  [redundant-expr]
+ xarray/plot/dataarray_plot.py: note: At top level:
+ xarray/plot/dataarray_plot.py: note: In function "_add_labels":
+ xarray/plot/dataarray_plot.py:1110: error: If condition is always true  [redundant-expr]
+ xarray/core/dataset.py: note: In member "expand_dims" of class "Dataset":
+ xarray/core/dataset.py:4576: error: If condition is always true  [redundant-expr]
+ xarray/core/dataset.py: note: In member "transpose" of class "Dataset":
+ xarray/core/dataset.py:6237: error: If condition is always false  [redundant-expr]
+ xarray/core/dataset.py: note: In member "dropna" of class "Dataset":
+ xarray/core/dataset.py:6378: error: If condition is always true  [redundant-expr]
+ xarray/core/dataset.py: note: In member "sortby" of class "Dataset":
+ xarray/core/dataset.py:8102: error: If condition is always true  [redundant-expr]
+ xarray/core/dataarray.py: note: In member "dot" of class "DataArray":
+ xarray/core/dataarray.py:5184: error: If condition is always true  [redundant-expr]
+ xarray/indexes/nd_point_index.py: note: In member "from_variables" of class "NDPointIndex":
+ xarray/indexes/nd_point_index.py:270: error: If condition is always false  [redundant-expr]
+ xarray/core/parallel.py: note: In function "check_result_variables":
+ xarray/core/parallel.py:50: error: If condition is always true  [redundant-expr]
+ xarray/core/parallel.py: note: At top level:
+ xarray/core/datatree.py: note: In member "__getitem__" of class "DataTree":
+ xarray/core/datatree.py:967: error: If condition is always true  [redundant-expr]
+ xarray/testing/assertions.py: note: In function "_assert_dataarray_invariants":
+ xarray/testing/assertions.py:431: error: If condition is always true  [redundant-expr]
+ xarray/testing/assertions.py: note: In function "_assert_dataset_invariants":
+ xarray/testing/assertions.py:466: error: If condition is always true  [redundant-expr]
+ xarray/testing/assertions.py: note: In function "_assert_internal_invariants":
+ xarray/testing/assertions.py:490: error: If condition is always true  [redundant-expr]
+ xarray/backends/common.py: note: In function "ensure_dtype_not_object":
+ xarray/backends/common.py:682: error: If condition is always false  [redundant-expr]
+ xarray/backends/common.py:682: error: If condition is always true  [redundant-expr]
+ xarray/computation/rolling.py: note: In member "construct" of class "Coarsen":
+ xarray/computation/rolling.py:1191: error: If condition is always true  [redundant-expr]
+ xarray/computation/rolling.py:1191: error: If condition is always false  [redundant-expr]
+ xarray/computation/rolling.py:1215: error: If condition is always false  [redundant-expr]
+ xarray/computation/fit.py: note: In function "polyfit":
+ xarray/computation/fit.py:186: error: If condition is always true  [redundant-expr]
+ xarray/tests/test_indexing.py: note: In member "test_group_indexers_by_index" of class "TestIndexers":
+ xarray/tests/test_indexing.py:104: error: If condition is always false  [redundant-expr]
+ xarray/tests/test_indexing.py: note: At top level:
+ xarray/tests/test_backends.py: note: In member "check_dtypes_roundtripped" of class "DatasetIOBase":
+ xarray/tests/test_backends.py:521: error: If condition is always false  [redundant-expr]
+ xarray/tests/test_backends.py: note: At top level:
+ xarray/tests/test_backends_datatree.py: note: In function "test_to_zarr_compute_false":
+ xarray/tests/test_backends_datatree.py:787: error: If condition is always true  [redundant-expr]

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
+ src/scikit_build_core/settings/skbuild_overrides.py:219: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:86: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:100: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:105: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:168: error: If condition is always true  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:249: error: If condition is always true  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:333: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:341: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:351: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:383: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:396: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/pyproject.py:408: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/_vendor/pyproject_metadata/__init__.py:369: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/setuptools/build_cmake.py:250: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/hatch/plugin.py:73: error: If condition is always false  [redundant-expr]
+ src/scikit_build_core/build/__main__.py:45: error: If condition is always true  [redundant-expr]

boostedblob (https://github.com/hauntsaninja/boostedblob)
+ boostedblob/boost.py:578: error: If condition is always true  [redundant-expr]
+ boostedblob/boost.py:590: error: If condition is always true  [redundant-expr]

nox (https://github.com/wntrblm/nox)
+ nox/sessions.py:617: error: If condition is always false  [redundant-expr]