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gh-77748: Document that float() accepts bytes-like objects #155501

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document that float() accepts bytes-like objects
Qanty228 Aug 10, 2026
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Delete duplicate news entry
Qanty228 Aug 11, 2026
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Delete duplicate news entry
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Merge branch 'python:main' into float-doc-fix
Qanty228 Aug 11, 2026
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document that float() accepts bytes-like objects (rst version)
Qanty228 Aug 11, 2026
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functions.rst trailing whitespace fix
Qanty228 Aug 11, 2026
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import array functions.rst float fix
Qanty228 Aug 11, 2026
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add more examples of float conversions
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Merge branch 'main' into float-doc-fix
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16 changes: 15 additions & 1 deletion Doc/library/functions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -778,7 +778,7 @@ are always available. They are listed here in alphabetical order.
single: NaN
single: Infinity

Return a floating-point number constructed from a number or a string.
Return a floating-point number constructed from a string, number, or bytes-like object, if possible.

Examples:

Expand All @@ -794,12 +794,26 @@ are always available. They are listed here in alphabetical order.
1000000.0
>>> float('-Infinity')
-inf
>>> float(b'3.2e2')
320.0
>>> float(bytearray(b'-2.5'))
-2.5
>>> float(memoryview(b'6.7'))
6.7
>>> import array
>>> float(array.array('b', [49, 50, 51]))
123.0

If the argument is a string, it should contain a decimal number, optionally
preceded by a sign, and optionally embedded in whitespace. The optional
sign may be ``'+'`` or ``'-'``; a ``'+'`` sign has no effect on the value
produced. The argument may also be a string representing a NaN
(not-a-number), or positive or negative infinity.

If the argument is a bytes-like object, its contents are parsed as a decimal
string. Therefore, the bytes-like object must contain a valid representation
of a floating-point number, otherwise :exc:`ValueError` is raised.

More precisely, the input must conform to the :token:`~float:floatvalue`
production rule in the following grammar, after leading and trailing
whitespace characters are removed:
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4 changes: 2 additions & 2 deletions Objects/clinic/floatobject.c.h
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4 changes: 2 additions & 2 deletions Objects/floatobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1560,12 +1560,12 @@ float.__new__ as float_new
x: object(c_default="NULL") = 0
/

Convert a string or number to a floating-point number, if possible.
Convert a string, number, or bytes-like object to a float, if possible.

@skirpichev skirpichev Aug 11, 2026

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According to the glossary, bytes-like is:

An object that supports the Buffer Protocol and can export a C-contiguous buffer.

That includes e.g. array.array(), which apparently not supported by the float constructor:

>>> import array
>>> array.array('i', [123])
array('i', [123])
>>> float(array.array('i', [123]))
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    float(array.array('i', [123]))
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: could not convert string to float: array('i', [123])

Please correct docstring, see the int()'s docstring as an example.

@Qanty228 Qanty228 Aug 11, 2026 •
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Thanks for the comment. A bytes-like object can be converted to a float if its contents are parsed as an ASCII
representation of a decimal number:

>>> import array
>>> array.array('b', [49, 50, 51])
array('b', [49, 50, 51])
>>> float(array.array('b', [49, 50, 51])) # '1', '2', '3'
123.0
>>> float(array.array('B', [32, 45, 49, 46, 49])) # ' ', '-', '1', '.', '1' 
-1.1
>>> float(array.array('B', [49, 101, 49])) # '1', 'e', '1'
10.0
>>> float(array.array('i', [49, 50, 51]))
Traceback (most recent call last):
  File "<python-input-5>", line 1, in <module>
    float(array.array('i', [49, 50, 51]))
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: could not convert string to float: array('i', [49, 50, 51])

[clinic start generated code]*/

static PyObject *
float_new_impl(PyTypeObject *type, PyObject *x)
/*[clinic end generated code: output=ccf1e8dc460ba6ba input=55909f888aa0c8a6]*/
/*[clinic end generated code: output=ccf1e8dc460ba6ba input=27eb1f7c62226a39]*/
{
if (type != &PyFloat_Type) {
if (x == NULL) {
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