Python: How to modify specific (key, value) pairs at list of dicts?

Question

I have:

l = [{"a": 2}, {"a": 4}, {"a": 10}, {"a": 11}]

I need to multiply by 2 all "a" dict keys. So I would have:

l = [{'a': 4}, {'a': 8}, {'a': 20}, {'a': 22}]

I can do it by such code:

for i in l:
    i.update({"a": 2 * i["a"]})

But it's ugly.

There should be nice Pythonic one-liner.

Answer 1

This is one liner code updating the dictionaries and returning the list. However, if dictionaries do not have a key it raises KeyError . And, all the dictionaries are updated until an exception occurs. I hope it helps but code readibility matters so simply updating dictionary in for loop does not cost much.

>>> l = [{"a": 2}, {"a": 4}, {"a": 10}, {"a": 11}]
>>> map(lambda x:(x,x.__setitem__("a",x["a"]*2))[0],l)
[{'a': 4}, {'a': 8}, {'a': 20}, {'a': 22}]

Answer 2

l = [{k:v*2} for d in l for k, v in d.items()]

Answer 3

This also takes into account if there actually is a key a .

for d in l:
  if 'a' in d:
    d['a'] *= 2

Or

for d in l:
  try:
    d['a'] *= 2
  except KeyError:
    # No key `a`
    pass