Build Dictionary in Python Loop - List and Dictionary Comprehensions

Question

I'm playing with some loops in python. I am quite familiar with using the "for" loop:

for x in y:     do something 

You can also create a simple list using a loop:

i = [] for x in y:    i.append(x) 

and then I recently discovered a nice efficient type of loop, here on Stack, to build a list (is there a name for this type of loop? I'd really like to know so I can search on it a little better):

[x.name for x in y] 

Ok, that being said, I wanted to go further with the last type of loop and I tried to build a python dictionary using the same type of logic:

{x[row.SITE_NAME] = row.LOOKUP_TABLE for row in cursor} 

instead of using:

x = {} for row in cursor:    x[row.SITE_NAME] = row.LOOKUP_TABLE 

I get an error message on the equal sign telling me it's an invalid syntax. I believe in this case, it's basically telling me that equal sign is a conditional clause (==), not a declaration of a variable.

My second question is, can I build a python dictionary using this type of loop or am I way off base? If so, how would I structure it?

Answer 1

The short form is as follows (called dict comprehension, as analogy to the list comprehension, set comprehension etc.):

x = { row.SITE_NAME : row.LOOKUP_TABLE for row in cursor } 

so in general given some _container with some kind of elements and a function _value which for a given element returns the value that you want to add to this key in the dictionary:

{ _key : _value(_key) for _key in _container } 

Answer 2

What you're using is called a list comprehension. They're pretty awesome ;)

They have a cousin called a generator expression that works like a list comprehension but instead of building the list all at once, they generate one item at a time. Hence the name generator. You can even build functions that are generators - there are plenty of questions and sites to cover that info, though.

You can do one of two things:

x = dict(((row.SITE_NAME, row.LOOKUP_TABLE) for row in cursor)) 

Or, if you have a sufficiently new version of Python, there is something called a dictionary comprehension - which works like a list comprehension, but produces a dictionary instead.

x = {row.SITE_NAME : row.LOOKUP_TABLE for row in cursor}