Python dict comprehension - access dict during creation

Question

I have a code like this:

dictionary = {}

for x in dict_of_x:
  if x['duration_to_cancel'] == None or x['duration_to_cancel'] > 5:
    key = x['key']
    value = x['value']

    if key not in dictionary or value > dictionary[key]:
      dictionary[key] = value

I want to make this code a dict comprehension:

dictionary = {x['key'] : x['value'] for x in dict_of_x \
            if ( True if x['duration_to_cancel'] == None else x['duration_to_cancel'] > 5)}

How can I include the second if statement in the 1st code box, to the dict comprehension?

Answer 1

Since all you are doing is a max you can use "max comprehension":

dict_of_x = (
    {'key':0,'value':0,'duration_to_cancel':10},
    {'key':0,'value':1,'duration_to_cancel':15},
    {'key':0,'value':2,'duration_to_cancel':1},
    {'key':1,'value':3,'duration_to_cancel':2},
    {'key':2,'value':4,'duration_to_cancel':None}
)

def yours():
    dictionary = {}
    for x in dict_of_x:
      if x['duration_to_cancel'] == None or x['duration_to_cancel'] > 5:
        key = x['key']
        value = x['value']
        if key not in dictionary or value > dictionary[key]:
          dictionary[key] = value
    return dictionary
print yours()

{0: 1, 2: 4}

def mine(): 
    dictionary = {key : max(x['value'] for x in dict_of_x if x['key'] == key) \
        for key in set(x['key'] for x in dict_of_x if x['duration_to_cancel'] == None or x['duration_to_cancel'] > 5)}
    return dictionary
print mine()

{0: 1, 2: 4}

Note however that list/dict/set/max comprehension just for the sake of it is not necessarily a good thing for readability. Also in you implementation you go through your dict only once, while my code will loop several times through it, which also comes with reduced speed:

%timeit yours()
1000000 loops, best of 3: 855 ns per loop


%timeit mine() 
100000 loops, best of 3: 2.32 µs per loop