ctypes Structure fields of type c_char_p or c_wchar_p used to accept strings with embedded null characters.  I noticed that Python 3.6.4 does refuse them.  It seems this has been changed in recent version(s).

There ARE use-cases for this:  The Windows-API OPENFILENAME structure is one example.  The Microsoft docs for the lpstrFilter field:

"""
lpstrFilter

    Type: LPCTSTR

    A buffer containing pairs of null-terminated filter strings. The last string in the buffer must be terminated by two NULL characters.
"""

I have attached a simple script which demonstrates this new behaviour; the output with Python 3.6.4 is this:

Traceback (most recent call last):
  File "nullchars.py", line 8, in <module>
    t.unicode = u"foo\0bar"
ValueError: embedded null character

PyUnicode_AsWideCharString was updated to raise ValueError for embedded nulls if the `size` output parameter is NULL. Z_set in cfield.c should be updated to get the size, which can be ignored here. For example:

    Py_ssize_t size; 
    buffer = PyUnicode_AsWideCharString(value, &size);

The change mentioned was made in GH-2462 for Issue13617 and was released in 3.6.3 (and 3.5.4 now in security-fix-only mode).

This is a regression. Eryk's solution LGTM. Do you mind to create a PR?

But u"foo\0bar" is not terminated by two NULL characters. If this is used in real code, it contains a bug. And the getter of this field will return the string only to the first null character. More work is needed for making this more reliable.