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> Yes, that is how it must work. This is symmetric with pure Python 
functions:
This is because in pure Python functions we have no enough information. I 
believe than origin expression is more useful in the help. Compare:
>>> import re, inspect
>>> p = re.compile('')
>>> p.match.__text_signature__
'(string, pos=0, endpos=sys.maxsize)'
>>> str(inspect.signature(p.match))
'(string, pos=0, endpos=2147483647)'
> I suspect the problem is, you misspelled one of the expressions you provided
> as a default value.
>>> import zlib, inspect
>>> zlib.decompress.__text_signature__
'(data, wbits=unspecified, bufsize=unspecified)'
>>> str(inspect.signature(zlib.decompress))
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1997, in wrap_value
 value = eval(s, module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2000, in wrap_value
 value = eval(s, sys_module_dict)
 File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 1468, in signature
 return Signature.from_builtin(obj)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2052, in from_builtin
 p(name, default)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2038, in p
 default_node = RewriteSymbolics().visit(default_node)
 File "/home/serhiy/py/cpython/Lib/ast.py", line 245, in visit
 return visitor(node)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2030, in visit_Name
 return wrap_value(node.id)
 File "/home/serhiy/py/cpython/Lib/inspect.py", line 2002, in wrap_value
 raise RuntimeError()
RuntimeError