[issue32500] PySequence_Length() raises TypeError on dict type

[Michał Górny]

New submission from Michał Górny <mgorny at gentoo.org>:

While debugging PyPy test failure on backports.lzma [1], I've noticed that PySequence_Check() on a dict type raises TypeError, e.g.:

  Traceback (most recent call last):
    File "test/test_lzma.py", line 273, in test_bad_args
      b"", format=lzma.FORMAT_RAW, filters={})
    File "/home/mgorny/git/backports.lzma/build/lib.linux-x86_64-3.6/backports/lzma/__init__.py", line 463, in decompress
      decomp = LZMADecompressor(format, memlimit, filters)
  TypeError: object of type 'dict' has no len()

The relevant C code is:

  static int
  parse_filter_chain_spec(lzma_filter filters[], PyObject *filterspecs)
  {
    Py_ssize_t i, num_filters;

    num_filters = PySequence_Length(filterspecs);
    ...

where filterspecs is the object corresponding to the {} dict in Python snippet.

According to the documentation [2], PySequence_Length() should be 'equivalent to the Python expression len(o).' The Python expression obviously does not raise TypeError:

  >>> len({})
  0

Therefore, I think that the behavior of PySequence_Length() is a bug, and the function should successfully return the dict length instead.

[1]:https://github.com/peterjc/backports.lzma
[2]:https://docs.python.org/3/c-api/sequence.html#c.PySequence_Length

----------
components: Extension Modules
messages: 309534
nosy: mgorny
priority: normal
severity: normal
status: open
title: PySequence_Length() raises TypeError on dict type
type: behavior
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32500>
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