Question

>>> def mod2(n):
...   print 'the remainder is', n % 2
... 
>>> mod2(5)
the remainder is 1
>>> mod2(2)
the remainder is 0
>>> mod2('%d')
the remainder is 2
>>> mod2('%d\rHELLO. I AM A POTATO!')
HELLO. I AM A POTATO!

Is there anyway to disable % symbol (operator.mod) from doing wacky string substitution stuff? I always use str.format if I need anything like that, and would generally rather this string substitution feature didn’t exist at all, giving a TypeError instead.

Answer 1

You can’t disable it with a switch, no. The str() type implements a __mod__ method to handle the formatting, it’s not that Python special-cased the expression just for strings.

As such, to prevent this you either need to cast the n argument to something that is not a string (by converting it to int() for example), or subclass str() to override the __mod__ method:

>>> class noformattingstr(str):
...     def __mod__(self, other):
...         raise TypeError('String formatting using "%" has been deprecated')
... 
>>> noformattingstr('hello world: %d') % 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __mod__
TypeError: String formatting using "%" has been deprecated

You can assign this to __builtins__.str, but this does not mean that all string literals will then use your subclass. You’d have to explicitly cast your str() values to noformattingstr() instances instead.