C-like assignment expression?

[Kay Schluehr]

Re: C-like assignment expression?

On 21 Mai, 11:38, boblat...@googl email.com wrote:

Hello,
>
I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match
>
...buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly. Just as ugly as first testing against the RE in the elif: clause
and then, if it matches, to re-evaluate the RE to access the match
groups.
>
Thanks,
robert

You are perfectly correct. Pythons design is lacking here IMO. But
what is your question?

[inhahe]

Re: C-like assignment expression?


<cokofreedom@gm ail.comwrote in message
news:06ea8b72-598e-4455-b3f9-e614ddb6ad94@p2 5g2000hsf.googl egroups.com...

On May 21, 4:57 pm, "inhahe" <inh...@gmail.c omwrote:

>one of the few things i miss from C is being able to use assignment in
>expressions. that's the only thing, really.
>also there's no switch/case, you have to use a dictionary of functions
>instead, although i rarely need that, usually i just use elif.

>
One thing I hate from C is the assignment in expressions...F orcing
myself to write
0 == Something
rather than
Something == 0

interesting trick, i've never thought of that/seen it
although if Python implemented it I think it should default to giving
warnings when you use = in an expression, that way you don't have to worry.

just to make sure I was mistakenly assigning values in statements is
annoying, it ruins the ease of reading.
>
I kind of agree with the select:case, but I think a key issue is how
to implement it. Elif is reasonable for now.
>
Diez, true I guess, but then we haven't seen what these expressions
are, and why there has to be three.

[inhahe]

Re: C-like assignment expression?

>>

>One thing I hate from C is the assignment in expressions...F orcing
>myself to write
>0 == Something
>rather than
>Something == 0

>
interesting trick, i've never thought of that/seen it
although if Python implemented it I think it should default to giving
warnings when you use = in an expression, that way you don't have to
worry.
>

That introduces complications though, do you want to see a pagefull of
warnings every time you import a module that uses the ='s?
You could specify in your python file that you want to suppress that
warning, but then you'd never know when you used = by accident when you
meant to use ==.
anyway i was thinking you could have a second assignment operator to use
just in expressions, and only allow that. it could be := since some
languages tend to use that. i wouldn't like it as a general assignment
operator but assignment in expressions is a special case. also <- or ->.
C uses -for functions but I think math/calculators use that for
assignment.

[MRAB]

Re: C-like assignment expression?

On May 21, 5:50 pm, "inhahe" <inh...@gmail.c omwrote:

One thing I hate from C is the assignment in expressions...F orcing
myself to write
0 == Something
rather than
Something == 0

>

interesting trick, i've never thought of that/seen it
although if Python implemented it I think it should default to giving
warnings when you use = in an expression, that way you don't have to
worry.

>
That introduces complications though, do you want to see a pagefull of
warnings every time you import a module that uses the ='s?
You could specify in your python file that you want to suppress that
warning, but then you'd never know when you used = by accident when you
meant to use ==.
anyway i was thinking you could have a second assignment operator to use
just in expressions, and only allow that. it could be := since some
languages tend to use that. i wouldn't like it as a general assignment
operator but assignment in expressions is a special case. also <- or ->.
C uses -for functions but I think math/calculators use that for
assignment.

My preference would be ?=.

if match ?= my_re1.match(li ne):
# use match
elif match ?= my_re2.match(li ne):
# use match
elif match ?= my_re3.match(li ne):
# use match

[sturlamolden]

Re: C-like assignment expression?

On May 21, 11:38 am, boblat...@googl email.com wrote:

if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match
>
...buy this is illegal in python.

Assignment expressions is disallowed in Python to protect against a
very common bug in C/C++ programs, the (accidental) confusion of

if (match = my_re1.match(li ne))

with

if (match == my_re1.match(li ne))

or vice versa.

[Paddy]

Re: C-like assignment expression?

On May 21, 10:38 am, boblat...@googl email.com wrote:

Hello,
>
I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match
>
...buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly. Just as ugly as first testing against the RE in the elif: clause
and then, if it matches, to re-evaluate the RE to access the match
groups.
>
Thanks,
robert

You could use named groups to search for all three patterns at once
like this:


original:

prog1 = re.compile(r'pa t1')
prog2 = re.compile(r'pa t2')
prog3 = re.compile(r'pa t3')
...

Becomes:

prog = re.compile(r'(? P<p1>pat1)|(?P< p2>pat2)|(?P<p3 >pat3)')
match = prog.match(line )
for p in 'p1 p2 p3'.split():
if match.groupdict ()[p]:
do_something_fo r_prog(p)


- Paddy.

[Kay Schluehr]

Re: C-like assignment expression?

On 21 Mai, 19:56, sturlamolden <sturlamol...@y ahoo.nowrote:

On May 21, 11:38 am, boblat...@googl email.com wrote:
>

if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match

>

...buy this is illegal in python.

>
Assignment expressions is disallowed in Python to protect against a
very common bug in C/C++ programs, the (accidental) confusion of
>
if (match = my_re1.match(li ne))
>
with
>
if (match == my_re1.match(li ne))
>
or vice versa.

This is just a syntactical issue. But what is the *value* of an
assigment? In Python it is always None: assigments are statements, not
expressions.

However Guido and team have found a *pragmatic* solution for this at
another place:

with open("myFile") as f:
BLOCK

Compare this with a possible syntactical form of an if-statement:

if EXPR as NAME:
BLOCK

This isn't ugly syntax-wise. It's just a bit harder to understand the
semantics of an if-statement. It might read like this:

"Evaluate EXPR and compute bool(EXPR). If this value is True assign
EXPR to NAME and execute BLOCK. Otherwise refuse both assigment and
BLOCK execution."

Maybe assignment can be performed unconditionally as in the C case.
I'm not sure about this.

[Antoon Pardon]

Re: C-like assignment expression?

On 2008-05-21, Diez B. Roggisch <deets@nospam.w eb.dewrote:

boblatest@googl email.com wrote:
>

>On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:
>>

>>Although that solution is pretty, it is not the canonical solution
>>because it doesn't cover the important case of "if" bodies needing to
>>access common variables in the enclosing scope. (This will be easier
>>in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
>>IMHO closer to a canonical solution to this FAQ.

>>
>Hello everybody,
>>
>thanks for the various answers. I'm actually pretty puzzled because I
>expected to see some obvious solution that I just hadn't found before.
>In general I find Python more elegant and syntactically richer than C
>(that's where I come from), so I didn't expect the solutions to be a
>lot more verbose and/or ugly (no offense) than the original idea which
>would have worked if Python's assignment statement would double as
>expression, as in C.

>
Well, it's a design-decision - and I'm pretty ok with it being a bit verbose
here - as it prevents a *great* deal of programming errors that would
otherwise happen from accidentally writing a = b where a == b was meant.

But that is an error that occurs because of the specific symbols chosen
to represent an assignment or equality test. At the time python was
first designed, other symbols for assignment were already used for
in other languages, like := and <-

So if preventing errors was the main motivation why not allow an
assignment to be an expression but use a symbol for the assignment
that would prevent those kind of errors?

I find it hard to believe that a design choice like whether or not
to have the assignment behave as an expression or not, was decided
on the ground of a particulare lexical representation of the assignment
symbol.

--
Antoon Pardon

[Paul McGuire]

Re: C-like assignment expression?

On May 21, 4:38 am, boblat...@googl email.com wrote:

Hello,
>
I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
  # use match
elsif (match = my_re2.match(li ne)):
  # use match
elsif (match = my_re3.match(li ne))
  # use match
>
...buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly. Just as ugly as first testing against the RE in the elif: clause
and then, if it matches, to re-evaluate the RE to access the match
groups.
>
Thanks,
robert

Try this.

-- Paul

class TestValue(objec t):
"""Class to support assignment and test in single operation"""
def __init__(self,v =None):
self.value = v

"""Add support for quasi-assignment syntax using '<<' in place of
'='."""
def __lshift__(self ,other):
self.value = other
return bool(self.value )


import re

tv = TestValue()
integer = re.compile(r"[-+]?\d+")
real = re.compile(r"[-+]?\d*\.\d+")
word = re.compile(r"\w +")

for inputValue in ("123 abc 3.1".split()):
if (tv << real.match(inpu tValue)):
print "Real", float(tv.value. group())
elif (tv << integer.match(i nputValue)):
print "Integer", int(tv.value.gr oup())
elif (tv << word.match(inpu tValue)):
print "Word", tv.value.group( )

Prints:

Integer 123
Word abc
Real 3.1

[Paul McGuire]

Re: C-like assignment expression?

This version is a bit better, since it follows the convention that
'<<' should return self.

class TestValue(objec t):
"""Class to support assignment and test in single operation"""
def __init__(self,v =None):
self.value = v

"""Add support for quasi-assignment syntax using '<<' in place of
'='."""
def __lshift__(self ,other):
self.value = other
return self

def __bool__(self):
return bool(self.value )
__nonzero__ = __bool__

-- Paul