TypeError: unsupported operand type(s) for |: 'list' and 'list'

Question

In Python, I want to write a list comprehension to iterate over the union of the keys for 2 dictionaries. Here's a toy example:

A = {"bruno":1, "abe":2}.keys()
B = {"abe":5, "carlton":10}.keys()

>>>[ k for k in A | B ]

I'm getting the error:

Traceback (most recent call last)
<ipython-input-221-ed92ac3be973> in <module>()
      2 B= {"abe":5, "carlton":10}.keys()
      3 
----> 4 [ k for k in A|B]

TypeError: unsupported operand type(s) for |: 'list' and 'list'

The comprehension works just fine for 1 dictionary. For example:

>>>[ k for k in A]
['bruno', 'abe']

Not sure where the error is. I'm following an example in a textbook and according to the book, this type of union and intersection operator should work fine. Please let me know your thoughts. Thank you.

Answer 1

In Python 2, dict.keys() is a list, not dictionary view. Use dict.viewkeys() instead:

A = {"bruno":1, "abe":2}.viewkeys()
B = {"abe":5, "carlton":10}.viewkeys()

[k for k in A | B]

Your example would have worked in Python 3, where the .keys() method has been changed to return a dictionary view by default.

Demo:

>>> A = {"bruno":1, "abe":2}.viewkeys()
>>> B = {"abe":5, "carlton":10}.viewkeys()
>>> [k for k in A | B]
['carlton', 'bruno', 'abe']

It sounds as if your textbook assumes you are using Python 3. Switch textbooks, or use Python 3 to run the examples, don't try to mix the two until you get a lot more experience with the differences between Python 2 and 3.

For the record, a dictionary view supports set operations with the |, ^, - and & operators against any iterable on the right-hand side; so the following works too:

A_dict = {"bruno":1, "abe":2}
B_dict = {"abe":5, "carlton":10}

[k for k in A_dict.viewkeys() | B_dict]