Python: Creating a dictionary using list comprehension from a list using lambda

Question

Conceptually, here is what I want to do:

lookup = {'A': 'aaa', 'B': 'bbb', 'C': 'ccc'}
keys = ['A', 'B', 'Z'] # Note 'Z' does not exist in dict
result = {}
for key in keys:
    if key in lookup:
        result[key] = lookup[key]
    else:
        result[key] = None

The above code yields the following, i.e.

# result == {'A': 'aaa', 'Z': None, 'B': 'bbb'}

Based on the advise in this post about dictionary comprehension syntax I am able to do the following using v2.6 syntax:

result = dict((lambda x: (x, lookup[x]) if x in lookup else (x, None))(key) for key in keys)

Which works and produces same results.

However, I was hoping that I would be able to do the following in v2.7 style (also mentioned in the above post). But that does not seem to work,

result = { (lambda x: x: a_dict[x] if x in a_dict else x: None)(key) for key in a_list }

I seem to be running afoul of use of : in lambda with that in denoting a key. I have also reviewed the examples given in the original PEP, too. They don't provide any guidance, either. Below is the output from command line:

>>> result = { (lambda x: x : a_dict[x] if x in a_dict else x: None)(key) for key in a_list }
  File "<stdin>", line 1
    result = { (lambda x: x : a_dict[x] if x in a_dict else x: None)(key) for key in a_list }
                            ^
SyntaxError: invalid syntax

What am I doing wrong? Is it possible to do what I am trying to do, i.e. use v2.7 syntax of dictionary comprehension with lambda?

Please note that I ask this only out of curiosity; it's not that I have a compulsion to write such production code, one that would require an expert to understand!

PS: I should mention two other related posts that I found helpful while trying do this. First is about using lambda in list comprehension. And another is creating a dictionary using list.

Answer 1

To do what you are trying with dictionary comprehension, you should make use of dict.get() method. Example -

>>> lookup = {'A': 'aaa', 'B': 'bbb', 'C': 'ccc'}
>>> keys = ['A', 'B', 'Z']
>>> result = {key:lookup.get(key) for key in keys}
>>> result
{'Z': None, 'A': 'aaa', 'B': 'bbb'}

dict.get() returns None if the key is not present in the dictionary. You can also specify a second argument to dict.get() which would be the default value returned, if the key you are trying to get is not present in the dictionary.

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To answer the question, you cannot use lambda to return something like a key:value pair in a dictionary comprehension (since the : syntax is not valid). If you want to do that, you should use dict() function with a generator expression , Example -

result = dict((lambda x: (x,a_dict[x]) if x in a_dict else (x,None))(key) for key in a_list)

or the more readable map() version -

result = dict(map((lambda x: (x,a_dict[x]) if x in a_dict else (x,None)), a_list)