filter items in a python dictionary where keys contain a specific string

Question

I'm a C coder developing something in python. I know how to do the following in C (and hence in C-like logic applied to python), but I'm wondering what the 'Python' way of doing it is.

I have a dictionary d, and I'd like to operate on a subset of the items, only those who's key (string) contains a specific substring.

i.e. the C logic would be:

for key in d:     if filter_string in key:         # do something     else         # do nothing, continue 

I'm imagining the python version would be something like

filtered_dict = crazy_python_syntax(d, substring) for key,value in filtered_dict.iteritems():     # do something 

I've found a lot of posts on here regarding filtering dictionaries, but couldn't find one which involved exactly this.

My dictionary is not nested and i'm using python 2.7

Answer 1

How about a dict comprehension:

filtered_dict = {k:v for k,v in d.iteritems() if filter_string in k} 

One you see it, it should be self-explanatory, as it reads like English pretty well.

This syntax requires Python 2.7 or greater.

In Python 3, there is only dict.items(), not iteritems() so you would use:

filtered_dict = {k:v for (k,v) in d.items() if filter_string in k} 

Answer 2

Go for whatever is most readable and easily maintainable. Just because you can write it out in a single line doesn't mean that you should. Your existing solution is close to what I would use other than I would user iteritems to skip the value lookup, and I hate nested ifs if I can avoid them:

for key, val in d.iteritems():     if filter_string not in key:         continue     # do something 

However if you realllly want something to let you iterate through a filtered dict then I would not do the two step process of building the filtered dict and then iterating through it, but instead use a generator, because what is more pythonic (and awesome) than a generator?

First we create our generator, and good design dictates that we make it abstract enough to be reusable:

# The implementation of my generator may look vaguely familiar, no? def filter_dict(d, filter_string):     for key, val in d.iteritems():         if filter_string not in key:             continue         yield key, val 

And then we can use the generator to solve your problem nice and cleanly with simple, understandable code:

for key, val in filter_dict(d, some_string):     # do something 

In short: generators are awesome.