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gh-142037: Improve error messages for printf-style formatting #142081

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gh-142037: Improve error messages for printf-style formatting
serhiy-storchaka Nov 29, 2025
8d9d009
Merge branch 'main' into str-format-errors
serhiy-storchaka Dec 1, 2025
c52feb2
Improve error message for '.
serhiy-storchaka Dec 1, 2025
0e43000
Try another format.
serhiy-storchaka Dec 1, 2025
c015d6a
Try another format.
serhiy-storchaka Dec 1, 2025
4e5538b
Merge branch 'str-format-errors' of github.com:serhiy-storchaka/cpyth…
serhiy-storchaka Jan 12, 2026
cb7ea11
Add prefix "format argument: " for a single argument.
serhiy-storchaka Jan 16, 2026
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20 changes: 10 additions & 10 deletions Lib/test/test_bytes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -781,16 +781,16 @@ def __int__(self):
pi = PseudoFloat(3.1415)

exceptions_params = [
('%x format: an integer is required, not float', b'%x', 3.14),
('%X format: an integer is required, not float', b'%X', 2.11),
('%o format: an integer is required, not float', b'%o', 1.79),
('%x format: an integer is required, not PseudoFloat', b'%x', pi),
('%x format: an integer is required, not complex', b'%x', 3j),
('%X format: an integer is required, not complex', b'%X', 2j),
('%o format: an integer is required, not complex', b'%o', 1j),
('%u format: a real number is required, not complex', b'%u', 3j),
('%i format: a real number is required, not complex', b'%i', 2j),
('%d format: a real number is required, not complex', b'%d', 2j),
('%x requires an integer, not float', b'%x', 3.14),
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@vstinner vstinner Jan 12, 2026

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I would prefer to keep the word "format", like:

Suggested change
('%x requires an integer, not float', b'%x', 3.14),
('%x format requires an integer, not float', b'%x', 3.14),

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@serhiy-storchaka serhiy-storchaka Jan 12, 2026

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I removed "format" because in "format argument 1: %x format requires an integer, not float" it would be repeated twice.

We can add the "format argument" prefix (without argument number or name) for the case when the operand is not a tuple or dict: "format argument: %x requires an integer, not float".

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@vstinner vstinner Jan 16, 2026

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The error message doesn't contain format argument 1: :

>>> "%x" % "abc"
TypeError: %x requires an integer, not str

In which case format argument 1: is added?

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@serhiy-storchaka serhiy-storchaka Jan 16, 2026 •
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When you have several format arguments. For example:

>>> "%x %s" % ("abc", 42)
TypeError: format argument 1: %x requires an integer, not str
>>> "%(a)x %(b)s" % {"a": "abc", "b": 42}
TypeError: format argument 'a': %x requires an integer, not str

We can add it for single non-tuple argument:

>>> "%x" % "abc"
TypeError: format argument: %x requires an integer, not str

What would be better?

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@vstinner vstinner Jan 16, 2026

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format argument: %x requires an integer, not str is better than just %x requires an integer, not str. It gives more context.

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@serhiy-storchaka serhiy-storchaka Jan 16, 2026

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Added. How is now?

('%X requires an integer, not float', b'%X', 2.11),
('%o requires an integer, not float', b'%o', 1.79),
(r'%x requires an integer, not .*\.PseudoFloat', b'%x', pi),
('%x requires an integer, not complex', b'%x', 3j),
('%X requires an integer, not complex', b'%X', 2j),
('%o requires an integer, not complex', b'%o', 1j),
('%u requires a real number, not complex', b'%u', 3j),
('%i requires a real number, not complex', b'%i', 2j),
('%d requires a real number, not complex', b'%d', 2j),
(
r'%c requires an integer in range\(256\)'
r' or a single byte, not .*\.PseudoFloat',
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239 changes: 197 additions & 42 deletions Lib/test/test_format.py
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19 changes: 12 additions & 7 deletions Lib/test/test_peepholer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -733,22 +733,27 @@ def test_format_errors(self):
with self.assertRaisesRegex(TypeError,
'not all arguments converted during string formatting'):
eval("'%s' % (x, y)", {'x': 1, 'y': 2})
with self.assertRaisesRegex(ValueError, 'incomplete format'):
with self.assertRaisesRegex(ValueError, 'stray % at position 2'):
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@vstinner vstinner Jan 12, 2026

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You might keep the word "format", like:

Suggested change
with self.assertRaisesRegex(ValueError, 'stray % at position 2'):
with self.assertRaisesRegex(ValueError, 'incomplete format: stray % at position 2'):

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@serhiy-storchaka serhiy-storchaka Jan 12, 2026

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I think that stray % is more common error than incomplete format.

eval("'%s%' % (x,)", {'x': 1234})
with self.assertRaisesRegex(ValueError, 'incomplete format'):
with self.assertRaisesRegex(ValueError, 'stray % at position 4'):
eval("'%s%%%' % (x,)", {'x': 1234})
with self.assertRaisesRegex(TypeError,
'not enough arguments for format string'):
eval("'%s%z' % (x,)", {'x': 1234})
with self.assertRaisesRegex(ValueError, 'unsupported format character'):
with self.assertRaisesRegex(ValueError,
'unsupported format %z at position 2'):
eval("'%s%z' % (x, 5)", {'x': 1234})
with self.assertRaisesRegex(TypeError, 'a real number is required, not str'):
with self.assertRaisesRegex(TypeError,
'format argument 1: %d requires a real number, not str'):
eval("'%d' % (x,)", {'x': '1234'})
with self.assertRaisesRegex(TypeError, 'an integer is required, not float'):
with self.assertRaisesRegex(TypeError,
'format argument 1: %x requires an integer, not float'):
eval("'%x' % (x,)", {'x': 1234.56})
with self.assertRaisesRegex(TypeError, 'an integer is required, not str'):
with self.assertRaisesRegex(TypeError,
'format argument 1: %x requires an integer, not str'):
eval("'%x' % (x,)", {'x': '1234'})
with self.assertRaisesRegex(TypeError, 'must be real number, not str'):
with self.assertRaisesRegex(TypeError,
'format argument 1: %f requires a real number, not str'):
eval("'%f' % (x,)", {'x': '1234'})
with self.assertRaisesRegex(TypeError,
'not enough arguments for format string'):
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22 changes: 11 additions & 11 deletions Lib/test/test_str.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1578,17 +1578,17 @@ def __int__(self):
self.assertEqual('%X' % letter_m, '6D')
self.assertEqual('%o' % letter_m, '155')
self.assertEqual('%c' % letter_m, 'm')
self.assertRaisesRegex(TypeError, '%x format: an integer is required, not float', operator.mod, '%x', 3.14)
self.assertRaisesRegex(TypeError, '%X format: an integer is required, not float', operator.mod, '%X', 2.11)
self.assertRaisesRegex(TypeError, '%o format: an integer is required, not float', operator.mod, '%o', 1.79)
self.assertRaisesRegex(TypeError, '%x format: an integer is required, not PseudoFloat', operator.mod, '%x', pi)
self.assertRaisesRegex(TypeError, '%x format: an integer is required, not complex', operator.mod, '%x', 3j)
self.assertRaisesRegex(TypeError, '%X format: an integer is required, not complex', operator.mod, '%X', 2j)
self.assertRaisesRegex(TypeError, '%o format: an integer is required, not complex', operator.mod, '%o', 1j)
self.assertRaisesRegex(TypeError, '%u format: a real number is required, not complex', operator.mod, '%u', 3j)
self.assertRaisesRegex(TypeError, '%i format: a real number is required, not complex', operator.mod, '%i', 2j)
self.assertRaisesRegex(TypeError, '%d format: a real number is required, not complex', operator.mod, '%d', 1j)
self.assertRaisesRegex(TypeError, r'%c requires an int or a unicode character, not .*\.PseudoFloat', operator.mod, '%c', pi)
self.assertRaisesRegex(TypeError, '%x requires an integer, not float', operator.mod, '%x', 3.14)
self.assertRaisesRegex(TypeError, '%X requires an integer, not float', operator.mod, '%X', 2.11)
self.assertRaisesRegex(TypeError, '%o requires an integer, not float', operator.mod, '%o', 1.79)
self.assertRaisesRegex(TypeError, r'%x requires an integer, not .*\.PseudoFloat', operator.mod, '%x', pi)
self.assertRaisesRegex(TypeError, '%x requires an integer, not complex', operator.mod, '%x', 3j)
self.assertRaisesRegex(TypeError, '%X requires an integer, not complex', operator.mod, '%X', 2j)
self.assertRaisesRegex(TypeError, '%o requires an integer, not complex', operator.mod, '%o', 1j)
self.assertRaisesRegex(TypeError, '%u requires a real number, not complex', operator.mod, '%u', 3j)
self.assertRaisesRegex(TypeError, '%i requires a real number, not complex', operator.mod, '%i', 2j)
self.assertRaisesRegex(TypeError, '%d requires a real number, not complex', operator.mod, '%d', 1j)
self.assertRaisesRegex(TypeError, r'%c requires an integer or a unicode character, not .*\.PseudoFloat', operator.mod, '%c', pi)

class RaisingNumber:
def __int__(self):
Expand Down
7 changes: 7 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-11-29-10-06-06.gh-issue-142037.OpIGzK.rst
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@@ -0,0 +1,7 @@
Improve error messages for printf-style formatting.
For errors in the format string, always include the position of the
start of the format unit.
For errors related to the formatted arguments, always include the number
or the name of the argument.
Raise more specific errors and include more information (type and number
of arguments, most probable causes of error).
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