Assignment Versus Equality

[BartC]

On 27/06/2016 00:22, Gregory Ewing wrote:
> BartC wrote:
>> On 26/06/2016 08:36, Lawrence D’Oliveiro wrote:
>>
>>> One of Python’s few mistakes was that it copied the C convention of
>>> using “=” for assignment and “==” for equality comparison.
>>
>> One of C's many mistakes. Unfortunately C has been very influential.
>
> I'm not sure it's fair to call it a mistake. C was
> designed for expert users, and a tradeoff was likely
> made based on the observation that assignment is
> used much more often than equality testing.

You mean the rationale was based on saving keystrokes?

A shame they didn't consider that when requiring parentheses around 
conditionals, semicolons, /*...*/ around comments, %d format codes and 
elaborate for-statements then!

But you might be right in that it was probably based on existing usage 
of Fortran, PL/I and maybe even Basic. (Its predecessor 'B' used "=", 
but B came through BCPL which I believe used ":="; perhaps the mistake 
was in discarding that.)

>> However, why couldn't Python have used "=" both for assignment, and
>> for equality?
>
> Because an expression on its own is a valid statement,
> so
>
>    a = b
>
> would be ambiguous as to whether it meant assigning b
> to a or evaluating a == b and discarding the result.

And that would be another reason why == is needed for equality.

-- 
Bartc