How do you translate this regular-expression idiom from Perl into Python?


Question

I switched from Perl to Python about a year ago and haven't looked back. There is only one idiom that I've ever found I can do more easily in Perl than in Python:

if ($var =~ /foo(.+)/) {
  # do something with $1
} elsif ($var =~ /bar(.+)/) {
  # do something with $1
} elsif ($var =~ /baz(.+)/) {
  # do something with $1
}

The corresponding Python code is not so elegant since the if statements keep getting nested:

m = re.search(r'foo(.+)', var)
if m:
  # do something with m.group(1)
else:
  m = re.search(r'bar(.+)', var)
  if m:
    # do something with m.group(1)
  else:
    m = re.search(r'baz(.+)', var)
    if m:
      # do something with m.group(2)

Does anyone have an elegant way to reproduce this pattern in Python? I've seen anonymous function dispatch tables used, but those seem kind of unwieldy to me for a small number of regular expressions...

1
45
9/23/2008 5:01:31 PM

Accepted Answer

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), we can now capture the condition value re.search(pattern, text) in a variable match in order to both check if it's not None and then re-use it within the body of the condition:

if match := re.search(r'foo(.+)', text):
  # do something with match.group(1)
elif match := re.search(r'bar(.+)', text):
  # do something with match.group(1)
elif match := re.search(r'baz(.+)', text)
  # do something with match.group(2)
3
4/28/2019 6:06:16 AM

Using named groups and a dispatch table:

r = re.compile(r'(?P<cmd>foo|bar|baz)(?P<data>.+)')

def do_foo(data):
    ...

def do_bar(data):
    ...

def do_baz(data):
    ...

dispatch = {
    'foo': do_foo,
    'bar': do_bar,
    'baz': do_baz,
}


m = r.match(var)
if m:
    dispatch[m.group('cmd')](m.group('data'))

With a little bit of introspection you can auto-generate the regexp and the dispatch table.


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