Reading from Python dict if key might not be present

user_155406 0
Viewed 63247

I am very new to Python and parsing data.

I can pull an external JSON feed into a Python dictionary and iterate over the dictionary. 

for r in results:
     print r['key_name']

As I walk through the results returned, I am getting an error when a key does not have a value (a value may not always exist for a record). If I print the results, it shows as 

'key_name': None, 'next_key':.................

My code breaks on the error. How can I control for a key not having a value?

Any help will be greatly appreciated!

Brock

python json
8 Answers

The preferred way, when applicable:

for r in results:
     print r.get('key_name')

this will simply print None if key_name is not a key in the dictionary. You can also have a different default value, just pass it as the second argument:

for r in results:
     print r.get('key_name', 'Missing: key_name')

If you want to do something different than using a default value (say, skip the printing completely when the key is absent), then you need a bit more structure, i.e., either:

for r in results:
    if 'key_name' in r:
        print r['key_name']

or

for r in results:
    try: print r['key_name']
    except KeyError: pass

the second one can be faster (if it's reasonably rare than a key is missing), but the first one appears to be more natural for many people.

user_958100

If possible, use the simplejson library for managing JSON data.

Daishiman_423450

the initial question in this thread is why I wrote the Dictor library, it handles JSON fallback and None values gracefully without needing try/except or If blocks.

Also gives you additional options like ignore upper/lower case,

see,

https://github.com/perfecto25/dictor

user_73274760

use has_key() , and that will return true or false

user_1797360

[Updated to remove careless mistake]

You could also do something like this:

for r in (row for row in results if 'a' in row):
    print r['a']

This uses a generator expression to pick "rows" out of "results" where "row" includes the key "a".

Here's a little test script:

results = [{'a':True}, {'b':True}, {'a':True}]
for r in (row for row in results if 'a' in row): print r['a']
user_130550
user_12650700
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