How to Return Dictionary Keys as a List in Python?

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Short answer: use the expression list(dict.keys()).

How to Return Dictionary Keys as a List in Python?

Problem Formulation

For example:

  • Given dictionary {'Alice': 18, 'Bob', 21, 'Carl': 24}
  • Return the keys as a list ['Alice', 'Bob', 'Carl']

Solution

  • The dict.keys() method returns a list of all keys in Python 2.
  • The dict.keys() method returns a dict_keys object in Python 3 that is a more efficient representation. To convert it to a list, use the built-in list() constructor like so: list(dict.keys())

You can see this in action here:

>>>
>>> d = {'Alice': 18, 'Bob': 21, 'Carl': 24}
>>> d.keys()
dict_keys(['Alice', 'Bob', 'Carl'])
>>> list(d.keys())
['Alice', 'Bob', 'Carl']

Having said this, you should ask yourself whether you really need to convert it to a list in the first place. Python iterators such as the dict_keys object have a great advantage in terms of memory usage—you don’t instantiate the whole list in memory but piggyback on the dictionary implementation of the keys. Not converting it to a list can save you significant memory and scarce processing overview.

For example, you could simply iterate over the dict_keys iterator rather than converting it to a list in the following example:

d = {'Alice': 18, 'Bob': 21, 'Carl': 24}

for key in d.keys():
    print('Key', key)

Compare this to the following version where you convert it to a list only for the purpose of iterating over it:

# This is not Pythonic!
for key in list(d.keys()):
    print('Key', key)

The output is the same in both cases:

Key Alice
Key Bob
Key Carl

But as the former method without conversion requires fewer characters and function calls, as well as fewer memory usage due to the saved list data structure, it is superior to the latter with list conversion.

On the other hand, the list data structure has some more advantages such as:

  • Lists can be indexed—such as in keys[-2] that accesses the second last key.
  • Lists can be slightly more efficient as generators at runtime because the i-th element doesn’t have to be generated but is already there. But this is an insignificant issue in the present scenario where you want to use the keys that are simple to iterate over—they are already there as well in the dictionary iterator.
  • Lists can capture the current state of the keys for future reference in a variable if the dictionary entries change over time.

Alternative Solution with Unpacking

The asterisk operator unpacks all values in an iterable into a higher-order data structure. For example, if you unpack the dictionary into a list using the [*dict] syntax, Python will place all keys into the dynamically created list environment. This creates a list data structure with all the dictionary keys.

The following example is analogous to the ones provided above—only using the asterisk operator for unpacking:

>>> d = {'Alice': 18, 'Bob': 21, 'Carl': 24}
>>> [*d]
['Alice', 'Bob', 'Carl']

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