Reading from Python dict if key might not be present
Question:
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
Answers:
use has_key() , and that will return true or false
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.
[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']
If possible, use the simplejson library for managing JSON data.
There are two straightforward ways of reading from Python dict if key might not be present. for example:
dicty = {'A': 'hello', 'B': 'world'}
- The pythonic way to access a key-value pair is:
value = dicty.get('C', 'default value')
- The non-pythonic way:
value = dicty['C'] if dicty['C'] else 'default value'
- even worse:
try:
value = dicty['C']
except KeyError as ke:
value = 'default value'
You can use the built in function hasattr
key='key_name'
# or loop your keys
if hasattr(e, key):
print(e[key])
else:
print('No key for %s' % key)
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,
Taken from https://stackoverflow.com/a/14923509/1265070
id = getattr(myobject, 'id', None)
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
use has_key() , and that will return true or false
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.
[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']
If possible, use the simplejson library for managing JSON data.
There are two straightforward ways of reading from Python dict if key might not be present. for example:
dicty = {'A': 'hello', 'B': 'world'}
- The pythonic way to access a key-value pair is:
value = dicty.get('C', 'default value')
- The non-pythonic way:
value = dicty['C'] if dicty['C'] else 'default value'
- even worse:
try:
value = dicty['C']
except KeyError as ke:
value = 'default value'
You can use the built in function hasattr
key='key_name'
# or loop your keys
if hasattr(e, key):
print(e[key])
else:
print('No key for %s' % key)
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,
Taken from https://stackoverflow.com/a/14923509/1265070
id = getattr(myobject, 'id', None)