Bitten by my C/Java experience

[Steven D'Aprano]

On Tuesday 05 May 2015 08:02, BartC wrote:

> On 04/05/2015 16:20, Cecil Westerhof wrote:
>> Potential dangerous bug introduced by programming in Python as if it
>> was C/Java. :-(
>> I used:
>>      ++tries
>> that has to be:
>>      tries += 1
> 
> I think I've come across that. It doesn't mind ++ so people are likely
> to be assume that increment works as in other languages.
> 
> I guess it just means +(+(a)).

Correct.


> But in that case, what meaning does:
> 
> a
> 
> or
> 
> a+b
> 
> have in Python? If they were function calls: a() or (a+b)(), then that's
> clear enough. But a+b doesn't do anything!

Not so.

The first one just does a name lookup and then throws the result away. The 
second one looks up names a and b, then adds them together, throwing away 
the result.

Here is one use for the first idiom:

try:
    bin
except NameError:
    # No built-in bin function, perhaps our Python is too old?
    def bin(num):
        ...


Here's a good use for the second:


def calculate(x):
    x + 0  # Fails if x is not a number.
    ...


That's an example of duck-typing. It allows any argument which supports 
addition with integers, e.g. x could be a number, or some kind of array or 
vector which supports addition with a scalar.


> (I think I would have picked up "++" and "--" as special tokens even if
> increment/decrement ops weren't supported. Just because they would
> likely cause errors through misunderstanding.)

Just because C made a mistake, doesn't mean other languages have to 
slavishly follow it.



-- 
Steven