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Date: Sat, 13 Oct 2012 20:03:42 +1100
Subject: Re: How to use "while" within the command in -c option of python?
From: Chris Angelico <rosuav@gmail.com>
To: python-list@python.org
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On Sat, Oct 13, 2012 at 7:41 PM, Thomas Bach
<thbach@students.uni-mainz.de> wrote:
> On Sat, Oct 13, 2012 at 12:32:41AM +0000, Steven D'Aprano wrote:
>>
>>  He gets SyntaxError because you can't follow a semicolon with a
>> statement that begins a block.
>
> Can someone provide a link on where to find this type of information?
> I was just hunting through =93The Python Language Reference=94 and could
> not find anything explicit. The only thing I found is
>
> http://docs.python.org/reference/simple_stmts.html
>
> =93Several simple statements may occur on a single line separated by
> semicolons.=94
>
> Anyways, this does not explicitly say =93You shall not put a compound
> statement after a simple statement separated by a semicolon.=94, right?

It's more that Python treats simple and compound statements as
completely separate beasts. You can combine simple statements on one
line, but compound statements mustn't be.

In my opinion, this is a major wart in Python syntax. You can argue
all you like about how it reduces code clarity to put these sorts of
things together, but that's a job for a style guide, NOT a language
requirement. Most code won't put an assignment followed by an
if/while/for, but when I'm working interactively, I often want to
recall an entire statement to edit and reuse, complete with its
initialization - something like (contrived example):

>>> a=3Dcollections.defaultdict(int)
>>> for x in open("RilvierRex.txt"): a[x]+=3D1

Then I keep doing stuff, keep doing stuff, and then come back to this
pair of lines. Since they're two lines, I have to recall them as two
separate entities, rather than as an initializer and the code that
uses it. Logically, they go together. Logically, they're on par with a
list comprehension, which initializes, loops, and assigns, all as a
single statement. But syntactically, they're two statements that have
to go on separate lines.

To force that sort of thing to be a single recallable statement, I can
do stupid tricks like:

>>> if True:
	a=3Dcollections.defaultdict(int)
	for x in open("RilvierRex.txt"): a[x]+=3D1

but that only works in IDLE, not in command-line interactive Python.

Note, by the way, that it's fine to put the statement _inside_ the for
on the same line. It's even legal to have multiple such statements:

>>> for x in (1,2,3): print(x); print(x);
1
1
2
2
3
3

If there's any ambiguity, it would surely be that, and not the simple
statement being first.

Okay, rant over. I'll go back to being nice now. :)

ChrisA