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Add informative notes to invariant function arguments (#3411)
Fixes #1115 and #3352 by providing a link to docs and a suggestion. This currently only takes care of 'List' and 'Dict', which are the most typical for this kind of errors.
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‎mypy/messages.py‎

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Original file line numberDiff line numberDiff line change
@@ -542,6 +542,7 @@ def incompatible_argument(self, n: int, m: int, callee: CallableType, arg_type:
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target = 'to {} '.format(name)
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msg = ''
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notes = [] # type: List[str]
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if callee.name == '<list>':
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name = callee.name[1:-1]
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n -= 1
@@ -610,7 +611,12 @@ def incompatible_argument(self, n: int, m: int, callee: CallableType, arg_type:
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arg_type_str = '**' + arg_type_str
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msg = 'Argument {} {}has incompatible type {}; expected {}'.format(
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n, target, arg_type_str, expected_type_str)
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if isinstance(arg_type, Instance) and isinstance(expected_type, Instance):
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notes = append_invariance_notes(notes, arg_type, expected_type)
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self.fail(msg, context)
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if notes:
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for note_msg in notes:
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self.note(note_msg, context)
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def invalid_index_type(self, index_type: Type, expected_type: Type, base_str: str,
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context: Context) -> None:
@@ -1345,6 +1351,33 @@ def pretty_or(args: List[str]) -> str:
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return ", ".join(quoted[:-1]) + ", or " + quoted[-1]
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def append_invariance_notes(notes: List[str], arg_type: Instance,
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expected_type: Instance) -> List[str]:
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"""Explain that the type is invariant and give notes for how to solve the issue."""
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from mypy.subtypes import is_subtype
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from mypy.sametypes import is_same_type
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invariant_type = ''
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covariant_suggestion = ''
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if (arg_type.type.fullname() == 'builtins.list' and
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expected_type.type.fullname() == 'builtins.list' and
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is_subtype(arg_type.args[0], expected_type.args[0])):
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invariant_type = 'List'
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covariant_suggestion = 'Consider using "Sequence" instead, which is covariant'
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elif (arg_type.type.fullname() == 'builtins.dict' and
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expected_type.type.fullname() == 'builtins.dict' and
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is_same_type(arg_type.args[0], expected_type.args[0]) and
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is_subtype(arg_type.args[1], expected_type.args[1])):
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invariant_type = 'Dict'
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covariant_suggestion = ('Consider using "Mapping" instead, '
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'which is covariant in the value type')
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if invariant_type and covariant_suggestion:
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notes.append(
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'"{}" is invariant -- see '.format(invariant_type) +
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'http://mypy.readthedocs.io/en/latest/common_issues.html#variance')
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notes.append(covariant_suggestion)
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return notes
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def make_inferred_type_note(context: Context, subtype: Type,
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supertype: Type, supertype_str: str) -> str:
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"""Explain that the user may have forgotten to type a variable.

‎test-data/unit/check-varargs.test‎

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Original file line numberDiff line numberDiff line change
@@ -593,3 +593,48 @@ class C:
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def foo(self) -> None: pass
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C().foo()
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C().foo(1) # The decorator's return type says this should be okay
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[case testInvariantDictArgNote]
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from typing import Dict, Sequence
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def f(x: Dict[str, Sequence[int]]) -> None: pass
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def g(x: Dict[str, float]) -> None: pass
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def h(x: Dict[str, int]) -> None: pass
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a = {'a': [1, 2]}
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b = {'b': ['c', 'd']}
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c = {'c': 1.0}
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d = {'d': 1}
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f(a) # E: Argument 1 to "f" has incompatible type Dict[str, List[int]]; expected Dict[str, Sequence[int]] \
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# N: "Dict" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance \
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# N: Consider using "Mapping" instead, which is covariant in the value type
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f(b) # E: Argument 1 to "f" has incompatible type Dict[str, List[str]]; expected Dict[str, Sequence[int]]
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g(c)
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g(d) # E: Argument 1 to "g" has incompatible type Dict[str, int]; expected Dict[str, float] \
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# N: "Dict" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance \
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# N: Consider using "Mapping" instead, which is covariant in the value type
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h(c) # E: Argument 1 to "h" has incompatible type Dict[str, float]; expected Dict[str, int]
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h(d)
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[builtins fixtures/dict.pyi]
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[case testInvariantListArgNote]
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from typing import List, Union
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def f(numbers: List[Union[int, float]]) -> None: pass
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a = [1, 2]
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f(a) # E: Argument 1 to "f" has incompatible type List[int]; expected List[Union[int, float]] \
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# N: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance \
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# N: Consider using "Sequence" instead, which is covariant
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x = [1]
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y = ['a']
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x = y # E: Incompatible types in assignment (expression has type List[str], variable has type List[int])
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[builtins fixtures/list.pyi]
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[case testInvariantTypeConfusingNames]
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from typing import TypeVar
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class Listener: pass
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class DictReader: pass
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def f(x: Listener) -> None: pass
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def g(y: DictReader) -> None: pass
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a = [1, 2]
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b = {'b': 1}
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f(a) # E: Argument 1 to "f" has incompatible type List[int]; expected "Listener"
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g(b) # E: Argument 1 to "g" has incompatible type Dict[str, int]; expected "DictReader"
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[builtins fixtures/dict.pyi]

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