Argument type checking in classical_cipher.py is buggy

[hoodmane]

Students often call these functions with strings. This results in an error like "str has no attribute parent":

    336         """
    337         dom = self.domain()  # = plaintext_space = ciphertext_space
--> 338         if not isinstance(M, StringMonoidElement) and M.parent() == dom:
    339             raise TypeError("Argument M (= %s) must be a string in the plaintext/ciphertext space." % M)
    340         from sage.rings.finite_rings.integer_mod import Mod

AttributeError: 'str' object has no attribute 'parent'

Note that it almost made it to raising a TypeError which would be tidier.

Component: cryptography

Branch: u/hoodmane/classical-ciper-typeerrors

Issue created by migration from https://trac.sagemath.org/ticket/33658

[hoodmane]

Branch: u/hoodmane/classical-ciper-typeerrors

[hoodmane]

Changed branch from u/hoodmane/classical-ciper-typeerrors to classical-ciper-typeerrors

[hoodmane]

Changed branch from classical-ciper-typeerrors to u/hoodmane/classical-ciper-typeerrors

[hoodmane]

comment:4

I get a permission error when trying to push though.

[sheerluck]

comment:5

Replying to @hoodmane:

I get a permission error when trying to push though.

try u/gh-hoodmane/classical-ciper-typeerrors

[yyyyx4]

Description changed:

--- 
+++ 
@@ -1,6 +1,6 @@
 Students often call these functions with strings. This results in an error like "str has no attribute parent":
 
-```py
+```python
     336         """
     337         dom = self.domain()  # = plaintext_space = ciphertext_space
 --> 338         if not isinstance(M, StringMonoidElement) and M.parent() == dom: