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Date: Thu, 02 Jan 2014 17:20:54 +0000
From: John Allsup <pydev@allsup.co>
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Subject: Ifs and assignments
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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):
	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