Can I have anonymous function with "static" variables in Python?

Question

For example

lambda x: re.compile(r'foobar').match(x)

is not so great, because it may recompile every time it is called (if re runs out of cache - thank you responders for pointing out the cache mechanism).

I can do this without recompiling:

def f(line):
    try:
        f.c
    except:
        f.c = re.compile(r'foobar')
    return f.c.match(line)

How to do it with a lambda, without recompiling?

And well, I don't want to use a helper function, to use inside the lambda. The whole point of using lambdas is "anonymity". So yes the lambda is anonymous, and self-contained.

re library has a caching mechanism, so your regex should not be compiled each time function is called. — Łukasz Rogalski
– Łukasz Rogalski, Commented Apr 23, 2015 at 13:47
In practice I wouldn't worry about that. The re module internally caches 100 regexes, so if you aren't using more than 100 patterns you won't see any significant performance gain from doing this — wim
– wim, Commented Apr 23, 2015 at 13:47
I'm mean if we assume that re is not caching (I think the OP just used re.compile as an example) — hitzg
– hitzg, Commented Apr 23, 2015 at 13:52
after you import re, you could set the MAXCACHE to whatever you like — wim
– wim, Commented Apr 23, 2015 at 23:51

chepner · Accepted Answer · 2015-04-23 14:14:49Z

11

The usual trick is to provide a default value for an argument you don't intend to supply.

lambda x, regexobject=re.compile(r'foobar'): regexobject.match(x)

The default value is evaluated when the lambda is defined, not each time it is called.

Rather than using the lambda, though, I would just define your regular expressions explicitly

regex1 = re.compile(r'foobar')
regex2 = re.compile(r'bazquux')
# etc

then pass the bound method around where needed. That is, instead of

somefunction(lambda x, regexobject=re.compile(r'foobar'): regexobject.match(x))

use

somefunction(regex1.match)

The use case for an anonymous function is one that will only be called once, so there is no sense in binding a name to it. The fact that you are concerned about re.compile being called multiple times indicates that the functions will be called several times.

edited Apr 23, 2015 at 14:14

answered Apr 23, 2015 at 14:07

chepner

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1 Comment

wim Over a year ago

+1 for the suggestion just to use the bound method. I see no advantage for trying to apply lambdas here

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