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NoneType handling for str.format() with specifiers #18952

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3fe1aa7
Check for None type in format call
VyZhu Apr 21, 2025
4180b55
Add check for alignment specifier on none
VyZhu Apr 21, 2025
49d8fbf
Fix alignment specifier check to be more dynamic
VyZhu Apr 21, 2025
c5d0b71
Move alignment check to special check function
VyZhu Apr 21, 2025
31542a1
Added unit tests to check for Error Message for None argument and als…
ChristinaTrinh Apr 21, 2025
30665c0
Merge pull request #1 from VallinZ/unitTest
ChristinaTrinh Apr 21, 2025
feda1d3
Attempt to recognize no __format__ functions must result in error
ChristinaTrinh Apr 21, 2025
5c4e701
Merge branch 'master' of https://github.com/VallinZ/mypy into unitTest
ChristinaTrinh Apr 21, 2025
2286f46
Deleted commented out code
ChristinaTrinh Apr 21, 2025
8b94be8
Remove print for test
VyZhu Apr 21, 2025
d4f70fb
Fix isinstance call with get_property_type
VyZhu Apr 21, 2025
e3d002e
Fix lint for alignment specifier on none
VyZhu Apr 21, 2025
c536cab
Merge pull request #3 from VallinZ/AddAlignmentSpecifierLogic
VallinZ Apr 22, 2025
693486e
Format changed by Linter
ChristinaTrinh Apr 24, 2025
cfc6b17
Fix merge conflicts
ChristinaTrinh Apr 24, 2025
c979af9
Extend check for format string specifiers and added more unit tests
ChristinaTrinh Apr 26, 2025
d46f562
Fix lint error
ChristinaTrinh Apr 26, 2025
35cf276
Merge pull request #5 from VallinZ/AddAlignmentSpecifierLogic
ChristinaTrinh Apr 26, 2025
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25 changes: 23 additions & 2 deletions mypy/checkstrformat.py
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Original file line number Diff line number Diff line change
Expand Up @@ -52,6 +52,7 @@
AnyType,
Instance,
LiteralType,
NoneType,
TupleType,
Type,
TypeOfAny,
Expand Down Expand Up @@ -98,7 +99,6 @@ def compile_new_format_re(custom_spec: bool) -> Pattern[str]:

# Conversion (optional) is ! followed by one of letters for forced repr(), str(), or ascii().
conversion = r"(?P<conversion>![^:])?"

# Format specification (optional) follows its own mini-language:
if not custom_spec:
# Fill and align is valid for all builtin types.
Expand All @@ -113,7 +113,6 @@ def compile_new_format_re(custom_spec: bool) -> Pattern[str]:
else:
# Custom types can define their own form_spec using __format__().
format_spec = r"(?P<format_spec>:.*)?"

return re.compile(field + conversion + format_spec)


Expand Down Expand Up @@ -338,6 +337,7 @@ def check_specs_in_format_call(

The core logic for format checking is implemented in this method.
"""

assert all(s.key for s in specs), "Keys must be auto-generated first!"
replacements = self.find_replacements_in_call(call, [cast(str, s.key) for s in specs])
assert len(replacements) == len(specs)
Expand Down Expand Up @@ -450,6 +450,27 @@ def perform_special_format_checks(
code=codes.STRING_FORMATTING,
)

if isinstance(get_proper_type(actual_type), NoneType):
# Perform type check of alignment specifiers on None
# If spec.format_spec is None then we use "" instead of avoid crashing
specifier_char = None
if spec.non_standard_format_spec and isinstance(call.args[-1], StrExpr):
arg = call.args[-1].value
specifier_char = next((c for c in (arg or "") if c in "<>^"), None)
elif isinstance(spec.format_spec, str):
specifier_char = next((c for c in (spec.format_spec or "") if c in "<>^"), None)

if specifier_char:
self.msg.fail(
(
f"Alignment format specifier "
f'"{specifier_char}" '
f"is not supported for None"
),
call,
code=codes.STRING_FORMATTING,
)

def find_replacements_in_call(self, call: CallExpr, keys: list[str]) -> list[Expression]:
"""Find replacement expression for every specifier in str.format() call.

Expand Down
31 changes: 31 additions & 0 deletions test-data/unit/check-formatting.test
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Original file line number Diff line number Diff line change
Expand Up @@ -265,6 +265,37 @@ b'%c' % (123)
-- ------------------


[case testFormatCallNoneAlignment]
from typing import Optional

'{:<1}'.format(None) # E: Alignment format specifier "<" is not supported for None
'{:>1}'.format(None) # E: Alignment format specifier ">" is not supported for None
'{:^1}'.format(None) # E: Alignment format specifier "^" is not supported for None
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Please add a few more tests - at least with f-strings, with conversion (e.g. "{!s:>5}".format(None) is allowed) and with dynamic spec (f"{None:{foo}}" is also valid, foo may be an empty string).

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@VallinZ VallinZ Apr 24, 2025 •
edited
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I'm looking at the case of

foo = "^"'
test2 = f"{None:{foo}}"

which python throws an error but the current implementation does not catch it.
I look into the callExp of this case, it return:

CallExpr:9(
  MemberExpr:9(
    StrExpr({:{}})
    format)
  Args(
    NameExpr(None [builtins.None])
    CallExpr:9(
      MemberExpr:9(
        StrExpr({:{}})
        format)
      Args(
        NameExpr(foo [mypy.LocalTest.test.foo])
        StrExpr()))))

Is there a way to see what value foo represent in MyPy? My understanding is that MyPy is a static tool, so it doesn't have the value of foo. So, technically, there is no way to check this case? So, would it be an option we just raise an warning that there might be an error rather saying that this is an error?

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would it be an option we just raise an warning

no, false positives should be avoided at all costs in mypy. If we can't statically prove that some code is invalid, mypy should be green.

There are Literal types and Instance.last_known_value field, representing the literal vale if we know it. However, we accept any dynamic specifiers everywhere else, so I think this should be deferred to a separate PR if you consider it important enough. For now I'd suggest just ignoring any dynamic spec components; formatting with dynamic alignment specifier should be a very rare case (IMO not worth supporting at all).


'{:<10}'.format('16') # OK
'{:>10}'.format('16') # OK
'{:^10}'.format('16') # OK

'{!s:<5}'.format(None) # OK
'{!s:>5}'.format(None) # OK
'{!s:^5}'.format(None) # OK

f"{None!s:<5}" # OK
f"{None!s:>5}" # OK
f"{None!s:^5}" # OK


f"{None:<5}" # E: Alignment format specifier "<" is not supported for None
f"{None:>5}" # E: Alignment format specifier ">" is not supported for None
f"{None:^5}" # E: Alignment format specifier "^" is not supported for None

my_var: Optional[str] = None
"{:<2}".format(my_var) # E: Alignment format specifier "<" is not supported for None
my_var = "test"
"{:>2}".format(my_var) # OK

[builtins fixtures/primitives.pyi]

[case testFormatCallParseErrors]
'}'.format() # E: Invalid conversion specifier in format string: unexpected }
'{'.format() # E: Invalid conversion specifier in format string: unmatched {
Expand Down