Python list comprehension for dictionaries in dictionaries?

Question

I just learned about list comprehension, which is a great fast way to get data in a single line of code. But something's bugging me.

In my test I have this kind of dictionaries inside the list:

[{'y': 72, 'x': 94, 'fname': 'test1420'}, {'y': 72, 'x': 94, 'fname': 'test277'}] 

The list comprehension s = [ r for r in list if r['x'] > 92 and r['x'] < 95 and r['y'] > 70 and r['y'] < 75 ] works perfectly on that (it is, in fact, the result of this line)

Anyway, I then realised I'm not really using a list in my other project, I'm using a dictionary. Like so:

{'test1420': {'y': '060', 'x': '070', 'fname': 'test1420'}} 

That way I can simply edit my dictionary with var['test1420'] = ...

But list comprehensions don't work on that! And I can't edit lists this way because you can't assign an index like that.

Is there another way?

Answer 1

You can do this:

s = dict([ (k,r) for k,r in mydict.iteritems() if r['x'] > 92 and r['x'] < 95 and r['y'] > 70 and r['y'] < 75 ]) 

This takes a dict as you specified and returns a 'filtered' dict.

Answer 2

If dct is

{'test1420': {'y': '060', 'x': '070', 'fname': 'test1420'},  'test277': {'y': 72, 'x': 94, 'fname': 'test277'},} 

Perhaps you are looking for something like:

[ subdct for key,subdct in dct.iteritems()    if 92<subdct['x']<95 and 70<subdct['y']<75 ] 

A little nicety is that Python allows you to chain inequalities:

92<dct[key]['x']<95 

instead of

if r['x'] > 92 and r['x'] < 95 

Note also that above I've written a list comprehension, so you get back a list (in this case, of dicts).

In Python3 there are such things as dict comprehensions as well:

{ n: n*n for n in range(5) } # dict comprehension {0: 0, 1: 1, 2: 4, 3: 9, 4: 16} 

In Python2 the equivalent would be

dict( (n,n*n) for n in range(5) ) 

I'm not sure if you are looking for a list of dicts or a dict of dicts, but if you understand the examples above, it is easy to modify my answer to get what you want.