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Date: Thu, 02 Jan 2014 16:57:26 -0800
From: Gary Herron <gary.herron@islandtraining.com>
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To: python-list@python.org
Subject: Re: Ifs and assignments
References: <52C59FF6.5000607@allsup.co> <52C5BD90.9020609@islandtraining.com> <52C5DDA6.5090207@allsup.co>
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On 01/02/2014 01:44 PM, John Allsup wrote:
> The point of my original post was that, whilst C's
>   if( x = 2 ) { do something }
> and
>   if( x == 2 ) { do something }
> are easy to confuse, and a source of bugs, having a construct like 
> follows:
>
> if x == 2:
>     do something # what happens at present
> if testFunc() as x:
>     do something with x
>
> using the 'as' syntax that appears with 'with' and 'except', would allow
> for the advantages of C style assignments in conditionals but without 
> the easy confusion, since here the syntax is significantly different 
> between assignment and equality testing (rather than a character apart 
> as happens with C).
>
> This occurs further down in my original post (past the point where you 
> inserted your reply).
>
> Another post suggested a workaround by defining a 'pocket' class, for 
> which I am grateful.
>
> John

Sorry.  I shot off my answer before reading the whole post.  That's 
never a good idea.


After reading to the end, I rather like your suggestion.  It works well 
in your example, , nicely avoids the C/C++ trap, and has some 
consistency with other parts of Python.

Gary Herron


>
>
> On 02/01/2014 19:27, Gary Herron wrote:
>> On 01/02/2014 09:20 AM, John Allsup wrote:
>>> Hi,
>>>
>>> This is my debut on this list.
>>>
>>> In many languages, such as C, one can use assignments in conditionals
>>> and expressions.  The most common, and useful case turns up when you
>>> have if/else if/else if/else constructs. Consider the following
>>> non-working pseudoPython.
>>>
>>> import re
>>> r1 = re.compile("hello (\d)")
>>> r2 = re.compile("world([!?])")
>>>
>>> w = "hello world!"
>>>
>>> if m = r1.search(w):
>>
>> This kind of thing in C/C+ has always been the source of much confusion
>> and potential errors, because the construct is so similar to an "=="
>> test.  Python does not replicate this potential for confusion. Instead,
>> we use two lines of code, an assignment and then the test.   If you find
>> that extra line of code inconvenient than at least you can take comfort
>> in the fact that it is clearer code.  If you are still not convinced,
>> ... then sorry, that's just the way Python is.
>>
>> Gary Herron
>>
>>
>>>     handleMatch1(m)
>>> elif m = r2.search(w):
>>>     handleMatch2(m)
>>> else:
>>>     print("No match")
>>>
>>> If the regular expressions are complex, running them multiple times
>>> (once to test, another to capture groups) isn't ideal.  On the other
>>> hand, at present, one has to either do:
>>>
>>> m = r1.search(w)
>>> if m:
>>>     handleMatch1(m)
>>> else:
>>>     m = r2.search(w)
>>>     if m:
>>>         handleMatch2(m)
>>>     else:
>>>         print("No match")
>>>
>>> if not running unnecessary matches, yet capturing groups in the event
>>> of a successful match, is what is desired.
>>>
>>> If there are multiple tests, the indentation gets silly.  This arises
>>> because having removed the ability to assign in an expression, there
>>> is no way to save the result of a function call that is used in a
>>> conditional at all.
>>>
>>> I am aware that this facility in C is a source of bugs, = being only a
>>> typo away from the more common ==.  With exceptions and contexts, we
>>> have:
>>>
>>> with open("file") as f:
>>>     doSomethingWith(f)
>>>
>>> try:
>>>     trySomething()
>>> except SomethingRandomGoingWrong as e:
>>>     lookAtException(e)
>>>
>>> What I am wondering is why not use a similar syntax with if, so that
>>> one could do
>>>
>>> if r1.search(w) as m:
>>>     g = m.groups()
>>>     print(g[1])
>>>
>>> This would remove the risk of errors by typos since the syntax for
>>> equality testing (if x == y:) is completely different from that for
>>> assigning in a conditional (which would look like 'if y as x:'
>>>
>>> Related would be to have Nonetype work with contexts such that
>>>
>>> with None as x:
>>>     doStuff(x)
>>>
>>> would do nothing.  This would allow things like:
>>>
>>> with maybeGetSomething as x:
>>>     doStuff(x)
>>>
>>> to call doStuff(x) within a context of maybeGetSomething returns
>>> something, or do nothing if nothing is returned.  (Adding an else-like
>>> keyword to with, or possibly using else in that context, would allow
>>> one to process a non-None object if returned, or else do something in
>>> response to a None object being returned by the maybeGetSomething.)
>>>
>>> Just a thought.
>>>
>>> Or what is the current 'Pythonic' way to do something like:
>>>
>>> if x = func1():
>>>     do1(x)
>>> elif x = func2():
>>>     do2(x)
>>> elif x = func3():
>>>     do3(x)
>>> elif x = func4():
>>>     do4(x)
>>> else:
>>>     do5()
>>>
>>> where each of func1,func2,func3,func4 have side effects so that func2
>>> is tested if and only if func1 returns a false value, func1 must be
>>> called only once, and what is returned from func1 must be available to
>>> the code inside the if block?
>>>
>>>
>>> John
>>>
>>
>