Efficient way to remove keys with empty strings from a dict

Question

Python 2.X

dict((k, v) for k, v in metadata.iteritems() if v)

---

Python 2.7 - 3.X

{k: v for k, v in metadata.items() if v}

Note that all of your keys have values. It's just that some of those values are the empty string. There's no such thing as a key in a dict without a value; if it didn't have a value, it wouldn't be in the dict.

It can get even shorter than BrenBarn's solution (and more readable I think)

{k: v for k, v in metadata.items() if v}

Tested with Python 2.7.3.

If you really need to modify the original dictionary:

empty_keys = [k for k,v in metadata.iteritems() if not v]
for k in empty_keys:
    del metadata[k]

Note that we have to make a list of the empty keys because we can't modify a dictionary while iterating through it (as you may have noticed). This is less expensive (memory-wise) than creating a brand-new dictionary, though, unless there are a lot of entries with empty values.

If you want a full-featured, yet succinct approach to handling real-world data structures which are often nested, and can even contain cycles, I recommend looking at the remap utility from the boltons utility package.

After pip install boltons or copying iterutils.py into your project, just do:

from boltons.iterutils import remap

drop_falsey = lambda path, key, value: bool(value)
clean = remap(metadata, visit=drop_falsey)

This page has many more examples, including ones working with much larger objects from Github's API.

It's pure-Python, so it works everywhere, and is fully tested in Python 2.7 and 3.3+. Best of all, I wrote it for exactly cases like this, so if you find a case it doesn't handle, you can bug me to fix it right here.

Based on Ryan's solution, if you also have lists and nested dictionaries:

For Python 2:

def remove_empty_from_dict(d):
    if type(d) is dict:
        return dict((k, remove_empty_from_dict(v)) for k, v in d.iteritems() if v and remove_empty_from_dict(v))
    elif type(d) is list:
        return [remove_empty_from_dict(v) for v in d if v and remove_empty_from_dict(v)]
    else:
        return d

For Python 3:

def remove_empty_from_dict(d):
    if type(d) is dict:
        return dict((k, remove_empty_from_dict(v)) for k, v in d.items() if v and remove_empty_from_dict(v))
    elif type(d) is list:
        return [remove_empty_from_dict(v) for v in d if v and remove_empty_from_dict(v)]
    else:
        return d