Question

What I want to do is simple:

collection = {'a':[], 'b':[], 'c':[]}
values = [1,2,3]

I want to make a function that produces the following: (append the values into the list element of the dictionary, the dic and the list are the same length)

{'a':[1], 'b':[2], 'c':[3]}

This is simple enough and I can do it using, a couple of for x in. But I want to do this in one line. (using two loops in the same line) and I can not get the syntax to work.

I have tried some things similar to this, but they all result in syntax error:

collection[c].append(value), for c in d.iteritems(), for value in values

Answer 1

You can’t do what you want to do on one line. You can create a new dictionary on one line though:

{k: collection[k] + [v] for k, v in zip(collection.keys(), values)}

Result:

>>> collection = {'a':[], 'b':[], 'c':[]}
>>> values = [1,2,3]
>>> {k: collection[k] + [v] for k, v in zip(collection.keys(), values)}
{'a': [1], 'c': [2], 'b': [3]}

This is called a dict comprehension. Like a list comprehension and a generator expression, you can use multiple loops in one of those, but you don’t need one here. zip() will pair up the keys from collection with the integers from values.

To modify a dict in-place, you’ll have to use 2 lines at least:

for k, v in zip(collection.keys(), values):
    collection[k].append(v)

Python does accept that on one line, but that goes against just about every styleguide I can look up for you:

for k, v in zip(collection.keys(), values): collection[k].append(v)

Python throws a syntax error because it interprets your line as a tuple of expressions (the commas make it a tuple), and two of your expressions are for statements, which cannot be used in an expression.