fix [feature] warn when passing str to something that accepts iterable[str] or sequence[str] #673 by asukaminato0721 · Pull Request #2293 · facebook/pyrefly

asukaminato0721 · 2026-02-03T09:04:05Z

Summary

Fixes #673

Implemented the new string-as-iterable warning and documented it, with checks that fire only when the argument is definitely str and the parameter is Iterable[str]/Sequence[str] without explicitly allowing str.

Test Plan

add test

yangdanny97

This seems to change the positions of a bunch of unrelated errors.

I think the change could be smaller and more self-contained.

For now, what if we didn't consider unions or assignability at all, and just made it error if the expected type is exactly Iterable[str] or Sequence[str] and the actual type is exactly str?

For the initial impl we probably don't even need to consider unions.

github-actions · 2026-02-03T17:01:19Z

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

packaging (https://github.com/pypa/packaging)
+  WARN src/packaging/_musllinux.py:84:28-72: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=src/packaging/_musllinux.py,line=84,col=28,endLine=84,endColumn=72,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
+  WARN src/scikit_build_core/builder/_load_provider.py:115:17-20: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN src/scikit_build_core/builder/_load_provider.py:118:59-62: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=src/scikit_build_core/builder/_load_provider.py,line=115,col=17,endLine=115,endColumn=20,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=src/scikit_build_core/builder/_load_provider.py,line=118,col=59,endLine=118,endColumn=62,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

pwndbg (https://github.com/pwndbg/pwndbg)
+  WARN pwndbg/commands/context.py:1508:23-32: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=pwndbg/commands/context.py,line=1508,col=23,endLine=1508,endColumn=32,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

meson (https://github.com/mesonbuild/meson)
+  WARN mesonbuild/backend/vs2010backend.py:1049:13-117: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ERROR mesonbuild/cmake/common.py:179:55-69: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+  WARN mesonbuild/cmdline.py:121:26-40: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN mesonbuild/cmdline.py:141:26-40: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN mesonbuild/interpreter/interpreter.py:2786:34-44: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN mesonbuild/mtest.py:2247:32-33: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:66:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:89:43-85: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:111:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:119:37-40: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:120:43-46: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:126:43-84: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:148:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:153:43-85: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:158:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:163:37-78: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:164:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:184:37-79: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:185:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:210:37-78: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:211:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:216:37-79: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:217:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:225:43-51: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:226:43-51: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:275:43-85: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:276:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:285:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:297:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:314:37-76: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:315:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:327:37-76: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:328:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:340:37-79: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:341:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:353:37-85: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:354:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:366:37-79: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:367:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:379:37-84: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:380:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:392:37-87: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:393:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:405:43-92: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:424:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:429:43-91: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:448:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:453:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:463:43-85: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN unittests/rewritetests.py:478:43-83: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=mesonbuild/backend/vs2010backend.py,line=1049,col=13,endLine=1049,endColumn=117,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::error file=mesonbuild/cmake/common.py,line=179,col=55,endLine=179,endColumn=69,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str
+ ::warning file=mesonbuild/cmdline.py,line=121,col=26,endLine=121,endColumn=40,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=mesonbuild/cmdline.py,line=141,col=26,endLine=141,endColumn=40,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=mesonbuild/interpreter/interpreter.py,line=2786,col=34,endLine=2786,endColumn=44,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=mesonbuild/mtest.py,line=2247,col=32,endLine=2247,endColumn=33,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=66,col=43,endLine=66,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=89,col=43,endLine=89,endColumn=85,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=111,col=43,endLine=111,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=119,col=37,endLine=119,endColumn=40,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=120,col=43,endLine=120,endColumn=46,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=126,col=43,endLine=126,endColumn=84,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=148,col=43,endLine=148,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=153,col=43,endLine=153,endColumn=85,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=158,col=43,endLine=158,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=163,col=37,endLine=163,endColumn=78,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=164,col=43,endLine=164,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=184,col=37,endLine=184,endColumn=79,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=185,col=43,endLine=185,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=210,col=37,endLine=210,endColumn=78,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=211,col=43,endLine=211,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=216,col=37,endLine=216,endColumn=79,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=217,col=43,endLine=217,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=225,col=43,endLine=225,endColumn=51,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=226,col=43,endLine=226,endColumn=51,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=275,col=43,endLine=275,endColumn=85,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=276,col=43,endLine=276,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=285,col=43,endLine=285,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=297,col=43,endLine=297,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=314,col=37,endLine=314,endColumn=76,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=315,col=43,endLine=315,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=327,col=37,endLine=327,endColumn=76,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=328,col=43,endLine=328,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=340,col=37,endLine=340,endColumn=79,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=341,col=43,endLine=341,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=353,col=37,endLine=353,endColumn=85,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=unittests/rewritetests.py,line=354,col=43,endLine=354,endColumn=83,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

... (truncated 13 lines) ...

zulip (https://github.com/zulip/zulip)
+  WARN zerver/lib/markdown/api_arguments_table_generator.py:234:17-66: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN zerver/lib/markdown/api_arguments_table_generator.py:241:17-87: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN zerver/lib/markdown/api_arguments_table_generator.py:246:17-248:18: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN zerver/management/commands/send_test_email.py:53:64-79: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN zerver/management/commands/send_test_email.py:57:80-95: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+ ERROR zerver/management/commands/send_test_email.py:65:67-84: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ::warning file=zerver/lib/markdown/api_arguments_table_generator.py,line=234,col=17,endLine=234,endColumn=66,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=zerver/lib/markdown/api_arguments_table_generator.py,line=241,col=17,endLine=241,endColumn=87,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=zerver/lib/markdown/api_arguments_table_generator.py,line=246,col=17,endLine=248,endColumn=18,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=zerver/management/commands/send_test_email.py,line=53,col=64,endLine=53,endColumn=79,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=zerver/management/commands/send_test_email.py,line=57,col=80,endLine=57,endColumn=95,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::error file=zerver/management/commands/send_test_email.py,line=65,col=67,endLine=65,endColumn=84,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str

schemathesis (https://github.com/schemathesis/schemathesis)
+ ERROR src/schemathesis/pytest/plugin.py:318:38-76: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ERROR src/schemathesis/pytest/plugin.py:335:42-88: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ERROR src/schemathesis/pytest/plugin.py:350:38-352:14: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ::error file=src/schemathesis/pytest/plugin.py,line=318,col=38,endLine=318,endColumn=76,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str
+ ::error file=src/schemathesis/pytest/plugin.py,line=335,col=42,endLine=335,endColumn=88,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str
+ ::error file=src/schemathesis/pytest/plugin.py,line=350,col=38,endLine=352,endColumn=14,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str

mkosi (https://github.com/systemd/mkosi)
+  WARN mkosi/config.py:4701:21-37: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN mkosi/config.py:4722:25-72: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN mkosi/config.py:4728:17-28: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=mkosi/config.py,line=4701,col=21,endLine=4701,endColumn=37,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=mkosi/config.py,line=4722,col=25,endLine=4722,endColumn=72,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=mkosi/config.py,line=4728,col=17,endLine=4728,endColumn=28,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

cwltool (https://github.com/common-workflow-language/cwltool)
+  WARN tests/test_toolargparse.py:218:26-45: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=tests/test_toolargparse.py,line=218,col=26,endLine=218,endColumn=45,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

dulwich (https://github.com/dulwich/dulwich)
+  WARN dulwich/contrib/swift.py:184:24-28: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=dulwich/contrib/swift.py,line=184,col=24,endLine=184,endColumn=28,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

vision (https://github.com/pytorch/vision)
+  WARN test/test_datasets.py:2479:29-85: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=test/test_datasets.py,line=2479,col=29,endLine=2479,endColumn=85,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

core (https://github.com/home-assistant/core)
+ ERROR homeassistant/components/androidtv_remote/media_player.py:186:45-55: No matching overload found for function `list.__init__` called with arguments: (str) [no-matching-overload]
+ ::error file=homeassistant/components/androidtv_remote/media_player.py,line=186,col=45,endLine=186,endColumn=55,title=Pyrefly no-matching-overload::No matching overload found for function `list.__init__` called with arguments: (str)%0A  Possible overloads:%0A  () -> None%0A  (iterable: Iterable[str], /) -> None [closest match]

setuptools (https://github.com/pypa/setuptools)
+  WARN setuptools/_vendor/packaging/_musllinux.py:84:28-72: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN setuptools/_vendor/tomli/_parser.py:342:32-48: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=setuptools/_vendor/packaging/_musllinux.py,line=84,col=28,endLine=84,endColumn=72,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=setuptools/_vendor/tomli/_parser.py,line=342,col=32,endLine=342,endColumn=48,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

werkzeug (https://github.com/pallets/werkzeug)
+ ERROR src/werkzeug/debug/__init__.py:382:49-77: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ::error file=src/werkzeug/debug/__init__.py,line=382,col=49,endLine=382,endColumn=77,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str

isort (https://github.com/pycqa/isort)
+  WARN isort/_vendored/tomli/_parser.py:256:32-48: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN isort/core.py:369:21-41: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=isort/_vendored/tomli/_parser.py,line=256,col=32,endLine=256,endColumn=48,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=isort/core.py,line=369,col=21,endLine=369,endColumn=41,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ ERROR mitmproxy/tools/console/master.py:186:73-76: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ::error file=mitmproxy/tools/console/master.py,line=186,col=73,endLine=186,endColumn=76,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str

dd-trace-py (https://github.com/DataDog/dd-trace-py)
+  WARN ddtrace/debugging/_encoding.py:166:32-36: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN tests/appsec/integrations/flask_tests/test_gunicorn_handlers.py:29:26-78: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=ddtrace/debugging/_encoding.py,line=166,col=32,endLine=166,endColumn=36,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=tests/appsec/integrations/flask_tests/test_gunicorn_handlers.py,line=29,col=26,endLine=29,endColumn=78,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

CPython (Argument Clinic) (https://github.com/python/cpython)
+ ERROR Tools/clinic/libclinic/codegen.py:161:24-37: No matching overload found for function `str.join` called with arguments: (str) [no-matching-overload]
+ ::error file=Tools/clinic/libclinic/codegen.py,line=161,col=24,endLine=161,endColumn=37,title=Pyrefly no-matching-overload::No matching overload found for function `str.join` called with arguments: (str)%0A  Possible overloads:%0A  (iterable: Iterable[LiteralString], /) -> LiteralString [closest match]%0A  (iterable: Iterable[str], /) -> str

pip (https://github.com/pypa/pip)
+  WARN src/pip/_internal/cli/spinners.py:54:44-54: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN src/pip/_vendor/packaging/_musllinux.py:84:28-72: Passing `str` to `Sequence[str]` treats the string as an iterable of characters [string-as-iterable]
+  WARN src/pip/_vendor/tomli/_parser.py:341:32-48: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=src/pip/_internal/cli/spinners.py,line=54,col=44,endLine=54,endColumn=54,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=src/pip/_vendor/packaging/_musllinux.py,line=84,col=28,endLine=84,endColumn=72,title=Pyrefly string-as-iterable::Passing `str` to `Sequence[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?
+ ::warning file=src/pip/_vendor/tomli/_parser.py,line=341,col=32,endLine=341,endColumn=48,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

sphinx (https://github.com/sphinx-doc/sphinx)
+  WARN sphinx/writers/latex.py:1974:17-65: Passing `str` to `Iterable[str]` treats the string as an iterable of characters [string-as-iterable]
+ ::warning file=sphinx/writers/latex.py,line=1974,col=17,endLine=1974,endColumn=65,title=Pyrefly string-as-iterable::Passing `str` to `Iterable[str]` treats the string as an iterable of characters%0A  Did you mean to pass an iterable of strings?

meta-codesync · 2026-02-04T18:50:36Z

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D92301565.

yangdanny97 · 2026-02-09T00:10:02Z

This is a lot of new errors, I'll need to think about how to restrict this so we don't give so many false positives. Additionally, it should probably be off by default

github-actions · 2026-02-23T00:42:10Z

This pull request has been automatically marked as stale because it has not had recent activity for more than 2 weeks.

If you are still working on this this pull request, please add a comment or push new commits to keep it active. Otherwise, please unassign yourself and allow someone else to take over.

Thank you for your contributions!

github-actions · 2026-03-31T00:45:33Z

This pull request has been automatically marked as stale because it has not had recent activity for more than 2 weeks.

If you are still working on this this pull request, please add a comment or push new commits to keep it active. Otherwise, please unassign yourself and allow someone else to take over.

Thank you for your contributions!

Copilot

Pull request overview

Adds a new diagnostic to Pyrefly that warns when a str is passed to parameters typed as Iterable[str] or Sequence[str], aiming to catch the common footgun where strings are treated as iterables of characters.

Changes:

Introduces ErrorKind::StringAsIterable (default severity warn) and emits it during argument type-checking when the pattern is detected.
Documents the new error kind and adds a regression test covering Iterable[str] / Sequence[str] and an explicit | str allowlist case.
Refactors overload calling to optionally retry without a hint when hint-driven checking produces call errors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
`website/docs/error-kinds.mdx`	Adds documentation for the new `string-as-iterable` diagnostic.
`pyrefly/lib/test/calls.rs`	Adds a test case asserting the new warning message for `Iterable[str]` and `Sequence[str]`.
`pyrefly/lib/alt/overload.rs`	Updates overload call flow to retry without hint when hint-driven call errors occur.
`pyrefly/lib/alt/callable.rs`	Implements the `string-as-iterable` warning and threads it through argument checking.
`crates/pyrefly_config/src/error_kind.rs`	Adds `StringAsIterable` to `ErrorKind` and assigns default severity `warn`.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Copilot · 2026-04-10T13:11:00Z

+    fn check_expr_argument(
+        &self,
+        expr: &Expr,
+        hint: &Type,
+        range: TextRange,
+        arg_errors: &ErrorCollector,
+        call_errors: &ErrorCollector,
+        tcc: &dyn Fn() -> TypeCheckContext,
+    ) -> (Type, bool) {
+        if hint.is_any() {
+            return (
+                self.expr_infer_type_info_with_hint(expr, None, arg_errors)
+                    .into_ty(),
+                false,
+            );
+        }
+        let got = self.expr_infer_type_info_with_hint(
+            expr,
+            Some(HintRef::new(hint, Some(call_errors))),
+            arg_errors,
+        );
+        let ok = self.check_type(got.ty(), hint, range, call_errors, tcc);
+        (got.into_ty(), ok)
+    }


check_expr_argument always returns the inferred type even when check_type fails. Previously expr_with_separate_check_errors would return want on failure (via check_and_return_type_info), which helps prevent cascading type errors and keeps downstream inference consistent. Consider restoring that behavior by returning hint (or got.with_ty(hint.clone())) when ok is false.

Copilot · 2026-04-10T13:11:00Z

+        if got.is_error() || got.is_any() || want.is_any() {
+            return;
+        }
+        let got_is_str = matches!(got, Type::ClassType(cls) if cls.is_builtin("str"));
+        if !got_is_str {
+            return;
+        }


The new warning only triggers when got is exactly Type::ClassType(str), so common cases like passing a string literal ("hello") or a LiteralString-typed value (both represented as literal-string types) won't warn even though they are definitely str at runtime. Consider treating got.is_literal_string() as str for this check (and add a test for the literal-literal case).

Copilot · 2026-04-10T13:11:00Z

+        // We want to use our hint to contextually type the arguments, but errors resulting
+        // from the hint should not influence overload selection. If there are call errors, we
+        // try again without a hint in case we can still match this overload.
+        let (call_errors, res, expected_types) = try_call(hint);
+        let (call_errors, res, expected_types) =
+            if tparams.is_some() && hint.is_some() && !call_errors.is_empty() {
+                try_call(None)
+            } else {
+                (call_errors, res, expected_types)
+            };


This overload-calling logic now may run callable_infer twice (first with a hint, then without) to avoid hint-driven errors affecting overload matching. Since this is a behavior/perf change beyond the new string-as-iterable warning, it would be good to mention it in the PR description (or split it) so the diagnostic/selection change is reviewed intentionally.

meta-cla Bot added the cla signed label Feb 3, 2026

This comment has been minimized.

Sign in to view

asukaminato0721 marked this pull request as draft February 3, 2026 09:38

This comment has been minimized.

Sign in to view

asukaminato0721 marked this pull request as ready for review February 3, 2026 13:06

yangdanny97 self-assigned this Feb 3, 2026

yangdanny97 requested changes Feb 3, 2026

View reviewed changes

This comment has been minimized.

Sign in to view

asukaminato0721 requested a review from yangdanny97 February 3, 2026 17:34

github-actions Bot added the stale label Feb 23, 2026

yangdanny97 closed this Mar 11, 2026

yangdanny97 reopened this Mar 11, 2026

github-actions Bot removed the stale label Mar 16, 2026

github-actions Bot added the stale label Mar 31, 2026

yangdanny97 removed their assignment Mar 31, 2026

asukaminato0721 added 4 commits April 10, 2026 21:55

fix

a52ad11

.

a27a8a7

update

e36024e

.

7707941

asukaminato0721 force-pushed the 673 branch from 0482786 to 7707941 Compare April 10, 2026 12:59

Copilot AI review requested due to automatic review settings April 10, 2026 12:59

github-actions Bot added the size/l label Apr 10, 2026

Copilot started reviewing on behalf of asukaminato0721 April 10, 2026 13:00 View session

Copilot AI reviewed Apr 10, 2026

View reviewed changes

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

fix [feature] warn when passing str to something that accepts iterable[str] or sequence[str] #673#2293

fix [feature] warn when passing str to something that accepts iterable[str] or sequence[str] #673#2293
asukaminato0721 wants to merge 4 commits intofacebook:mainfrom
asukaminato0721:673

asukaminato0721 commented Feb 3, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

yangdanny97 left a comment

Uh oh!

This comment has been minimized.

github-actions Bot commented Feb 3, 2026

Uh oh!

meta-codesync Bot commented Feb 4, 2026

Uh oh!

yangdanny97 commented Feb 9, 2026

Uh oh!

github-actions Bot commented Feb 23, 2026

Uh oh!

github-actions Bot commented Mar 31, 2026

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Apr 10, 2026

Uh oh!

Copilot AI Apr 10, 2026

Uh oh!

Copilot AI Apr 10, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

Conversation

asukaminato0721 commented Feb 3, 2026

Summary

Test Plan

Uh oh!

This comment has been minimized.

This comment has been minimized.

yangdanny97 left a comment

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

github-actions Bot commented Feb 3, 2026

Uh oh!

meta-codesync Bot commented Feb 4, 2026

Uh oh!

yangdanny97 commented Feb 9, 2026

Uh oh!

github-actions Bot commented Feb 23, 2026

Uh oh!

github-actions Bot commented Mar 31, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Apr 10, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Apr 10, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Apr 10, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants