Open files in "rock&roll" mode

Question

I'm wondering what is going on with the file open() mode validation (Python2.7):

>>> with open('input.txt', 'illegal') as f:
...     for line in f:
...         print line
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: mode string must begin with one of 'r', 'w', 'a' or 'U', not 'illegal'

>>> with open('input.txt', 'rock&roll') as f:
...     for line in f:
...         print line
... 
1

2

3

So, I cannot open the file in illegal mode, but I can open it in rock&roll mode. What mode is actually used for opening the file in this case?

Note that on python3 I cannot use both illegal and rock&roll:

>>> with open('input.txt', 'rock&roll') as f:
...     for line in f:
...         print(line)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid mode: 'rock&roll'
>>> with open('input.txt', 'illegal') as f:
...     for line in f:
...         print(line)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid mode: 'illegal'

And, this is confusing, why the behavior is different for python3.x?

Answer 1

The Python 2.x open function essentially delegates its work to the C library fopen function. On my system, the documentation for fopen contains:

The argument mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):

Your ock&roll is considered "additional characters".

In Python 3, the allowed open modes are more restricted (essentially, only valid strings are permitted).