Why = = (and not just =)

[Hans Nowak]

Joe Green wrote:
> Sorry, I cant help aking stupid questions:
> 
> I understand why we need = = in C, but why in Python (or Java),
> surely if you write
>  if a=b: pass # syntax error
> it could not possibly mean anything other than what I intended
> (which was of course if a = = b:) ?

But it leads to ambiguities in other situations... a = b = c, for example.

Interestingly, very old versions of Python did use = as the equality operator. 
  See Misc/HISTORY in the source distribution:

New features in 0.9.6:
[...]
- '==' is now the only equality operator; "../demo/scripts/eqfix.py" is
   a script that fixes old Python modules

Release 0.9.3 (never made available outside CWI)
[...]
- C comparison operators: == != (the old = and <> remain valid).

ABC apparently used = as well, and <> for inequality.  I'm assuming this is 
where the first versions of Python got it from.  Judging from the history file, 
= was soon replaced by ==; <> is still valid today, but deprecated in favor of !=.

Cheers,

-- 
Hans (hans at zephyrfalcon.org)
http://zephyrfalcon.org/