More general and simple solution is to make tempfile.NamedTemporaryFile new-style class.
Old-style class:
>>> import tempfile
>>> f = tempfile.NamedTemporaryFile()
>>> len(f)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/serhiy/py/cpython-2.7/Lib/tempfile.py", line 391, in __getattr__
a = getattr(file, name)
AttributeError: 'file' object has no attribute '__len__'
New-style class:
>>> import tempfile
>>> f = tempfile.NamedTemporaryFile()
>>> len(f)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type '_TemporaryFileWrapper' has no len()