gh-151303 : Improve `SyntaxError` suggestions for common operator typos and cross-language mistakes by Aniketsy · Pull Request #151375 · python/cpython

Aniketsy · 2026-06-11T17:46:06Z

Issue: Improve SyntaxError suggestions for common operator typos and cross-language mistakes #151303

StanFromIreland · 2026-06-13T09:40:18Z

+    | invalid_eqeqeq

+invalid_diamond_op:
+    | a='<' b='>' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax.  Maybe you meant '!=' instead of '<>'?") }


Won't this break when __future__.CO_FUTURE_BARRY_AS_BDFL is true?

hmm , yes

aniket@DESKTOP-074O80J:/mnt/d/cpython/cpython$ ./python.exe -c " __futu> from __future__ import barry_as_FLUFL > 1 < > 2 > " File "<string>", line 3 1 < > 2 ^^^ SyntaxError: invalid syntax. Maybe you meant '!=' instead of '<>'?

please let me know your thoughts on this ...

aniket@DESKTOP-074O80J:/mnt/d/cpython/cpython$ git diff Grammar/python.gram diff --git a/Grammar/python.gram b/Grammar/python.gram index 61a6d5ea90..19ef406437 100644 --- a/Grammar/python.gram +++ b/Grammar/python.gram @@ -805,7 +805,11 @@ compare_op_bitwise_or_pair[CmpopExprPair*]: | invalid_eqeqeq invalid_diamond_op: - | a='<' b='>' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '!=' instead of '<>'?") } + | a='<' b='>' { + (p->flags & PyPARSE_BARRY_AS_BDFL) + ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '<>' instead of '< >'?") + : RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '!=' instead of '<>'?") + } eq_bitwise_or[CmpopExprPair*]: '==' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, Eq, a) } invalid_eqeqeq: | a='==' b='=' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' instead of '==='?") }

output:

aniket@DESKTOP-074O80J:/mnt/d/cpython/cpython$ ./python.exe -c "1 < > 2" File "<string>", line 1 1 < > 2 ^^^ SyntaxError: invalid syntax. Maybe you meant '!=' instead of '<>'?

aniket@DESKTOP-074O80J:/mnt/d/cpython/cpython$ ./python.exe -c " __futu> from __future__ import barry_as_FLUFL > 1 < > 2 > " File "<string>", line 3 1 < > 2 ^^^ SyntaxError: invalid syntax. Maybe you meant '<>' instead of '< >'?

hmm, yes

No, this example doesn't break __future__ import. But I suspect, that __future__.CO_FUTURE_BARRY_AS_BDFL is the reason that your code doesn't work without spaces. This can be "fixed" by this patch:

diff --git a/Tools/build/generate_token.py b/Tools/build/generate_token.py index 9ee5ec86e75..28a0ce1ff78 100755 --- a/Tools/build/generate_token.py +++ b/Tools/build/generate_token.py @@ -178,7 +178,7 @@ def generate_chars_to_token(mapping, n=1): def make_c(infile, outfile='Parser/token.c'): tok_names, ERRORTOKEN, string_to_tok = load_tokens(infile) - string_to_tok['<>'] = string_to_tok['!='] +# string_to_tok['<>'] = string_to_tok['!='] chars_to_token = {} for string, value in string_to_tok.items(): assert 1 <= len(string) <= 3

Look at the difference:

$ cat a.py 1 =! 2 $ cat a.py | ./python -m tokenize 1,0-1,1: NUMBER '1' 1,2-1,3: OP '=' 1,3-1,4: OP '!' 1,5-1,6: NUMBER '2' 1,6-1,7: NEWLINE '\n' 2,0-2,0: ENDMARKER ''

vs

$ cat a.py 1 <> 2 $ cat a.py | ./python -m tokenize 1,0-1,1: NUMBER '1' 1,2-1,4: OP '<>' 1,5-1,6: NUMBER '2' 1,6-1,7: NEWLINE '\n' 2,0-2,0: ENDMARKER ''

Regardless on a joke, it looks as a tokenizer bug. I'll open a separate issue.

thanks for looking into this, i'll revert the changes

No, this example doesn't break future import.

It still does break it, we're suggesting != for < > which is invalid syntax when __future__.CO_FUTURE_BARRY_AS_BDFL is true.

Yes, this is wrong, see my comment below. But nothing to do with __future__.barry_as_FLUFL.

Currently, we have a hidden token (<> is alias for !=) in the grammar. So, this patch will not produce an error without a space between. See #151464.

serhiy-storchaka

Like in the previous PR, check that there is no gap between tokens.

serhiy-storchaka · 2026-06-14T07:46:34Z


+    def test_diamond_operator(self):
+        self._check_error(
+            "1 < > 2",


There should not be space in the middle.

Aniketsy · 2026-06-15T03:47:02Z

@skirpichev thanks for the review, i've updated with helper function

skirpichev · 2026-06-17T08:08:30Z

+        if (strcmp(tok_str, "<>") == 0) {
+            RAISE_SYNTAX_ERROR("invalid syntax.  Are you trying to overthrow the SC?  Use operator \"!=\"!");
+            return -1;
+        }


I would prefer not use this workaround for #151464. Leave "<>" case for another PR.

Should i revert the changes for this case ?

This is my suggestion, yes.

I would prefer not use this workaround for #151464. Leave "<>" case for another PR.

@skirpichev are you planning to work on this PR as issue was assigned to you, or i can open a follow up pr after this . please let me know 😊

Feel free to do this.

I'm waiting for opinion of core developers. Beware that posted patch is only a partial solution. Proper support of this easter egg will require much more work. Do we need this? I'm not sure. Other options:

call this not a bug

remove joke.

I'm waiting for opinion of core developers.

in that case i think i should also wait for the opinion.

Beware that posted patch is only a partial solution. Proper support of this easter egg will require much more work.

and meanwhile, i'll explore this, and will update you in the issue ... Thanks :)

serhiy-storchaka · 2026-06-17T09:00:51Z

We wait for other PR to be merged first.

…e-151303.mzlJxi.rst

serhiy-storchaka · 2026-06-28T05:01:58Z

For future -- never use rebase, amend and force-push. This forces us to review the entire PR from the beginning, instead of just the new changes.

Aniketsy · 2026-06-28T05:29:25Z

For future -- never use rebase, amend and force-push. This forces us to review the entire PR from the beginning, instead of just the new changes.

sorry for creating extra works, i didn't know we can solve conflicts without rebase, i'll keep in mind and i always wanted to never use rebase as i used to mess with this in past and this leads to creating my PR dirty that's the reason i built grebase(https://github.com/Aniketsy/grebase) But now i think i got alternative way to do . Thanks!

Aniketsy · 2026-07-14T13:00:32Z

gentle ping, i've resolved the conflict in parser.c, i think it would be great if we can move forward with this . Thanks :)

Aniketsy requested review from lysnikolaou and pablogsal as code owners June 11, 2026 17:46

bedevere-app Bot added the awaiting review label Jun 11, 2026

bedevere-app Bot mentioned this pull request Jun 11, 2026

Improve SyntaxError suggestions for common operator typos and cross-language mistakes #151303

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Aniketsy commented Jun 13, 2026

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Aniketsy mentioned this pull request Jun 14, 2026

gh-146495: Improve SyntaxError message for && and || operators #150906

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serhiy-storchaka reviewed Jun 14, 2026

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skirpichev requested changes Jun 15, 2026

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bedevere-app Bot added awaiting core review and removed awaiting review labels Jun 15, 2026

skirpichev reviewed Jun 17, 2026

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skirpichev requested review from StanFromIreland and serhiy-storchaka June 17, 2026 08:11

Aniketsy and others added 10 commits June 27, 2026 18:47

Improve SyntaxError suggestions for common operator typos

c56b1d0

add question mark in the suggestion and tests

a0d9269

update tests

a99fb0b

📜🤖 Added by blurb_it.

4d1a4b7

improve syntaxerror message <> typo

7479668

Update Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-03-41-39.gh-issu…

86d3df7

…e-151303.mzlJxi.rst

improve syntaxerror message <> typo

231949c

Improve SyntaxError suggestions for common operator typos

118dbf2

fix and update tests

148d8b9

add helper function, update tests

7a5f540

revert <> error message, update tests

ef3de3f

Aniketsy force-pushed the fix-151303 branch from d89dfcf to ef3de3f Compare June 27, 2026 19:41

skirpichev approved these changes Jun 28, 2026

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Aniketsy added 2 commits July 14, 2026 12:35

resolve conflicts

c92b786

regenerate parser.c

b0cad50

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serhiy-storchaka commented Jun 17, 2026

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Aniketsy commented Jun 11, 2026 •

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