Iterate over dictionary of class containing a list

Question

I have something like this:

class C:
  def get_data(): returns a list

d = {several instances of C}

Now, how do I iterate over all elements of all lists? I failed miserably:

[e for e in [v.get_data() for _, v in d.items()]]

The inner loop will produce a list of lists which can not be digested by the outer loop.

What is the outer loop doing? Don't you just want `[v.get_data() for v in d.values()]`. What should the output be? Please give a [mcve]. — jonrsharpe, May 27 '17 at 11:16
If you are using python2.7, I would recommend using iteritems/itervalues, as they require less memory. See this https://stackoverflow.com/questions/13998492/iteritems-in-python. — otadmor, May 27 '17 at 11:41

score 4 · Accepted Answer · answered May 27 '17 at 11:35

4

You want a flatten:

[e for _, v in d.items() for e in v.get_data()]

Note the order of the loops.

answered May 27 '17 at 11:35

Dan D.

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piokuc · Answer 2 · 2017-05-27T11:31:02.370

3

import itertools

class C:
  def get_data(self): return ['a','b']

d = {'a' : C(), 'b' : C()}

print [e for e in itertools.chain.from_iterable(v.get_data() for v in d.values())]

Will print

['a', 'b', 'a', 'b']

edited May 27 '17 at 11:31

answered May 27 '17 at 11:25

piokuc

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score 2 · Answer 3 · answered May 27 '17 at 11:57

2

Just modify your existing expression -

[item for sublist in [v.get_data() for _, v in d.items()] for item in sublist]

answered May 27 '17 at 11:57

Amey Dahale

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pylang · Answer 4 · 2017-05-27T11:58:35.247

1

@piokuc's answer is the better option, but in Python 3, yield from is a possibility:

class C:
    def get_data(self): 
        return ['a','b']


def extract(dict_):
    """Yield elements from sublists."""
    for _, v in dict_.items():
        yield from v.get_data()


d = {'a' : C(), 'b' : C()}
[e for e in extract(d)]
# ['a', 'b', 'a', 'b']

edited May 27 '17 at 11:58

answered May 27 '17 at 11:50

pylang

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martineau · Answer 5 · 2017-05-27T14:01:57.533

Not sure what you mean by "iterate over all elements", but based on the code in your question, the following will create a list of all the elements in both Python 2 & 3 and doesn't require importing anything else:

class C:
    def get_data(self): return ['a', 'b']

d = {'a' : C(), 'b' : C()}

print([e for c in d.values() for e in c.get_data()])  # -> ['a', 'b', 'a', 'b']

In Python 2 you might want to use d.itervalues() instead of d.values() because it creates an iterator of the values rather than creating a temporary list of them (which is what Python 3 does automatically when the values() method called).

Iterate over dictionary of class containing a list

5 Answers5