Enable checking for f strings

[nielsbuwen]

Resolves #17714 , fixes #11723

Support for f-string checking was already partially implemented. Consider this code:

f"{123:abc}"
"{:abc}".format(123)

Mypy ran these lines through the format checker. The call expressions look like this:

# f-string
CallExpr:1(
  MemberExpr:1(
    StrExpr({:{}})
    format)
  Args(
    IntExpr(123)
    StrExpr(abc)))

# format
CallExpr:2(
  MemberExpr:2(
    StrExpr({:abc})
    format)
  Args(
    IntExpr(123)))

The difference is, that the actually static "abc" is converted to a "dynamic" spec.

This MR attempts to enable type checking of f-strings by inlining these "semi dynamic" specs.

Open Questions:

• is this a good idea? Or should the actual parsing of f-strings be changed?
• the test i added always passes on my machine: pytest mypy/test/testcheck.py::TypeCheckSuite::check-string-format.test. It never finds any errors even when it should clearly fail

Checklist

• Read the Contributing Guidelines
• Add tests for all changed behavior
• Make sure CI passes
• Do not force push to PR once reviewed

[A5rocks]

According to mypy-issues, this PR also fixes:

• #11723

(I just realized after I ran it that that issue is mentioned in 17714 but whatever.)

[A5rocks]

IMO this should go to test-data/unit/check-formatting.test

[A5rocks]

Would you mind adding a test case? I imagine just "{:{}}".format("aaaa") would work... (I'm not sure what could possibly trigger this though)

[A5rocks]

I assume this happens if the resulting format string isn't a valid format string. Is that caught elsewhere, or does that deserve a self.msg.fail here?

[sterliakov]

the test i added always passes on my machine

Yep, [builtins ...] should go after the case body. Now you defined three empty tests with some text in between. (Ough, the README is really a bit confusing regarding this - I'll open a documentation PR to improve the language there).

[case acceptFstringWithoutSpecs]
reveal_type(f"{123} {True} {1 + 2} {'foo'}")  # N: Revealed type is "builtins.str"
[builtins fixtures/f_string.pyi]

[github-actions]

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

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/connector.py:1344:36: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int")  [str-format]
+ aiohttp/connector.py:1344:36: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-str-format for more info

colour (https://github.com/colour-science/colour)
+ colour/plotting/phenomena.py:328: error: Incompatible types in string interpolation (expression has type "ndarray[tuple[Any, ...], dtype[floating[_16Bit] | floating[_32Bit] | float64]]", placeholder has type "int | float")  [str-format]
+ colour/plotting/phenomena.py:656: error: Incompatible types in string interpolation (expression has type "Buffer", placeholder has type "int | float")  [str-format]
+ colour/plotting/phenomena.py:656: error: Incompatible types in string interpolation (expression has type "_SupportsArray[dtype[Any]] | _NestedSequence[_SupportsArray[dtype[Any]]] | complex | bytes | str | _NestedSequence[complex | bytes | str]", placeholder has type "int | float")  [str-format]

async-utils (https://github.com/mikeshardmind/async-utils)
+ src/async_utils/lockout.py:78: error: Unrecognized format specification "!r"  [str-format]
+ src/async_utils/lockout.py:151: error: Unrecognized format specification "!r"  [str-format]

core (https://github.com/home-assistant/core)
+ homeassistant/components/stiebel_eltron/climate.py:146: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/tests/scalar/test_na_scalar.py:38: error: Unrecognized format specification "xxx"  [str-format]

dulwich (https://github.com/dulwich/dulwich)
+ dulwich/patch.py:479: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int")  [str-format]

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/sync/messages.py:89: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]

materialize (https://github.com/MaterializeInc/materialize)
+ misc/python/materialize/feature_benchmark/benchmark_result.py:93: error: Incompatible types in string interpolation (expression has type "T", placeholder has type "int | float")  [str-format]
+ misc/python/materialize/feature_benchmark/benchmark_result.py:102: error: Incompatible types in string interpolation (expression has type "T", placeholder has type "int | float")  [str-format]

freqtrade (https://github.com/freqtrade/freqtrade)
+ freqtrade/persistence/trade_model.py:878: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/rpc/telegram.py:438: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/rpc/telegram.py:460: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/strategy/interface.py:1638: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]

ignite (https://github.com/pytorch/ignite)
+ ignite/utils.py:134: error: Incompatible types in string interpolation (expression has type "Number", placeholder has type "int | float")  [str-format]
+ ignite/utils.py:165: error: Incompatible types in string interpolation (expression has type "Number", placeholder has type "int | float")  [str-format]