Convert [key1,val1,key2,val2] to a dict?

by 
b = dict(zip(a[::2], a[1::2]))

If a is large, you will probably want to do something like the following, which doesn’t make any temporary lists like the above.

from itertools import izip
i = iter(a)
b = dict(izip(i, i))

In Python 3 you could also use a dict comprehension, but ironically I think the simplest way to do it will be with range() and len(), which would normally be a code smell.

b = {a[i]: a[i+1] for i in range(0, len(a), 2)}

So the iter()/izip() method is still probably the most Pythonic in Python 3, although as EOL notes in a comment, zip() is already lazy in Python 3 so you don’t need izip().

i = iter(a)
b = dict(zip(i, i))

In Python 3.8 and later you can write this on one line using the “walrus” operator (:=):

b = dict(zip(i := iter(a), i))

Otherwise you’d need to use a semicolon to get it on one line.

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