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feat: support constant folding in StringFormatterChecker.checkers_for_c_type [1/1] #19978

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feat: support constant folding in StringFormatterChecker.checkers_for_c_type [1/1] #19978

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30d454d
[mypyc] feat: support constant folding in `StringFormatterChecker.che…
BobTheBuidler Oct 2, 2025
d30b2b5
Update checkstrformat.py
BobTheBuidler Oct 2, 2025
cb38f60
Update checkstrformat.py
BobTheBuidler Oct 2, 2025
6548995
Update checkstrformat.py
BobTheBuidler Oct 2, 2025
44e9353
add check tests
BobTheBuidler Oct 4, 2025
f7567fd
Merge branch 'master' into constant-fold-checkers-for-c-type
BobTheBuidler Oct 6, 2025
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9 changes: 7 additions & 2 deletions mypy/checkstrformat.py
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@
import mypy.errorcodes as codes
from mypy import message_registry
from mypy.checker_shared import TypeCheckerSharedApi
from mypy.constant_fold import constant_fold_expr
from mypy.errors import Errors
from mypy.maptype import map_instance_to_supertype
from mypy.messages import MessageBuilder
Expand Down Expand Up @@ -1005,8 +1006,12 @@ def check_expr(expr: Expression) -> None:
and len(expr.value) != 1
):
self.msg.requires_int_or_single_byte(context)
elif isinstance(expr, (StrExpr, BytesExpr)) and len(expr.value) != 1:
self.msg.requires_int_or_char(context)
elif isinstance(folded := constant_fold_expr(expr, "<unused>"), str):
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note to self: can I get cur_mod_id from context?

if len(folded) != 1:
self.msg.requires_int_or_char(context)
elif isinstance(expr, BytesExpr):
if len(expr.value) != 1:
self.msg.requires_int_or_char(context)

return check_expr, check_type

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25 changes: 19 additions & 6 deletions test-data/unit/check-formatting.test
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Expand Up @@ -103,14 +103,27 @@ a = None # type: Any
[typing fixtures/typing-medium.pyi]

[case testStringInterpolationC]
from typing import Final

final_int: Final = 1
final_float: Final = 1.0
final_char_string: Final = 's'
final_empty_string: Final = ''
final_string: Final = 'ab'

'%c' % 1
'%c' % 1.0 # E: "%c" requires int or char (expression has type "float")
'%c' % final_int
'%c' % 1.0 # E: "%c" requires int or char (expression has type "float")
'%c' % final_float # E: "%c" requires int or char (expression has type "float")
'%c' % 's'
'%c' % '' # E: "%c" requires int or char
'%c' % 'ab' # E: "%c" requires int or char
'%c' % b'a' # E: "%c" requires int or char (expression has type "bytes")
'%c' % b'' # E: "%c" requires int or char (expression has type "bytes")
'%c' % b'ab' # E: "%c" requires int or char (expression has type "bytes")
'%c' % final_char_string
'%c' % '' # E: "%c" requires int or char
'%c' % final_empty_string # E: "%c" requires int or char
'%c' % 'ab' # E: "%c" requires int or char
'%c' % final_string # E: "%c" requires int or char
'%c' % b'a' # E: "%c" requires int or char (expression has type "bytes")
'%c' % b'' # E: "%c" requires int or char (expression has type "bytes")
'%c' % b'ab' # E: "%c" requires int or char (expression has type "bytes")
[builtins fixtures/primitives.pyi]

[case testStringInterpolationMappingTypes]
Expand Down
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