Dictionaries

Python's efficient key/value hash table structure is called a dict. The contents of a dict can be written as a series of key: value pairs within braces { }, e.g. dict = {key1:value1, key2:value2, ... }. The "empty dict" is just an empty pair of curly braces {}.

Looking up or setting a value in a dict uses square brackets, e.g. dict['foo'] looks up the value under the key 'foo'. Strings, numbers, and tuples work as keys, and any type can be a value. Other types may or may not work correctly as keys (strings and tuples work cleanly since they are immutable). Looking up a value which is not in the dict throws a KeyError -- use in to check if the key is in the dict, or use dict.get(key) which returns the value or None if the key is not present (or get(key, not-found) allows you to specify what value to return in the not-found case).

type=codeblock|id=py3_dict_init_and_lookup|autocreate=python3
## Can build up a dict by starting with the the empty dict {}
## and storing key/value pairs into the dict like this:
## dict[key] = value-for-that-key
dict = {}
dict['a'] = 'alpha'
dict['g'] = 'gamma'
dict['o'] = 'omega'
 
print(dict)  ## {'a': 'alpha', 'o': 'omega', 'g': 'gamma'}
 
print(dict['a'])     ## Simple lookup, returns 'alpha'
dict['a'] = 6        ## Put new key/value into dict
print('a' in dict)   ## True
## print(dict['z'])                  ## Throws KeyError
if 'z' in dict: print(dict['z'])     ## Avoid KeyError
print(dict.get('z'))  ## None (instead of KeyError)