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Date: Fri, 22 Feb 2013 10:21:27 +1100
Subject: Re: Python Newbie
From: Chris Angelico <rosuav@gmail.com>
To: python-list@python.org
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On Fri, Feb 22, 2013 at 9:40 AM,  <piterrr.dolinski@gmail.com> wrote:
> Thanks to all for quick relies.
>
> Chris, you are (almost) spot on with the if blocks indentation. This is w=
hat I do, and it has served me well for 15 years.
>
> code
> code
>
>    if (some condition)
>    {
>       code
>       code
>    }
>

I understand you have the freedom to do that in most languages, but
that's simply a restriction of Python: you have to use a more
conventional indentation system. Personally, I don't see the 'if' as
being special here, and the body of it (which IS special, as it may or
may not be executed) is already standing out.

> I am nervous about using variables "out of the blue", without having to d=
eclare them. For example, when I write "i =3D 0" it is perfectly OK to Pyth=
on without 'i' being declared earlier. How do I know that I haven't used th=
is variable earlier and I am unintentionally overwriting the value? I find =
I constantly have to use the search facility in the editor, which is not fu=
n.
>
> You see, Javascript, for one, behaves the same way as Python (no variable=
 declaration) but JS has curly braces and you know the variable you have ju=
st used is limited in scope to the code within the { }. With Python, you ha=
ve to search the whole file.

That's a fair point. The elision of variable declarations is a design
decision that Python is unlikely to reverse, and it happens to be one
I disagree with. But it's there, so when I code Python, I accept it :)
There are other languages similar to Python that have strict variable
declarations, and with them the awesomeness that is infinitely nested
scopes; Python just has global (module) scope and function scope, plus
'nonlocal' which can solve some problems (but I've yet to find it do
so without damaging code clarity).

> What about Python's ambiguity?
> For example, in C you would write
>
> if (myVar !=3D 0)
>   do something
>
> in Python, this is legal
>
> if (not myVar):
>   do something
>
> What does this mean? Is it a test for myVar being equal to zero or a test=
 for null, or else?

Python has a state called "truthiness". Generally speaking, something
with content is true, and something without content is false. The None
singleton, empty lists, empty tuples, the integer 0, and a regular
expression result that didn't match, all evaluate as False in a
boolean context; most other things evaluate as True. If you
specifically care if it's not zero, you ask if it's not zero; if you
want to know if it's falsy, you check "not X". The facilities are
there to do whatever you want.

ChrisA