Dictionaries¶
Dictionaries are unordered collections that represent mappings from keys to values. Dictionaries are indexed by their keys. Indexing a dictionary with a certain key returns the value stored at that key (or raises an error if that key isn’t present in the dictionary). Dictionary keys are unique!
Important
Only hashable data can be used as dictionary keys. The specifics are beyond the scope of this course, but it basically means that only immutable data can be keys. For example, a tuple can be a key, but a list cannot. A tuple containing a list cannot, because that list itself could be mutated.
Attempting to use unhashable data as a key will result in an error.
Making a Dictionary¶
Sets can be constructed with a dictionary literal:
my_dict = {key1: value1, key2: value2, key3: value3}
Note that using empty braces ({}) creates an empty dictionary, not an empty set.
The dict Constructor¶
Dictionaries can also be created using the dict constructor.
To use this, you can pass either no arguments, another dictionary, or an iterable
collection where each item is an iterable with two items representing (key, value)
pairs. With each of these options, you can also provide additional keyword arguments:
my_dict1 = dict()
my_dict2 = dict(key1=value1, key2=value2)
my_dict3 = dict(my_dict2)
my_dict4 = dict([(key1, value1), (key2, value2)])
Iterating Over a Dictionary¶
Important!!!
When iterating over a dictionary with a for loop, the loop variable will always
be a key in the dictionary. The iteration order is determined by the order in which
key-value pairs were inserted into the dictionary.
If you want to iterate over a dictionary’s values, you can do:
for value in my_dict.values():
...
If you want to iterate over the key-value pairs within a dictionary, you can instead do:
for key, value in my_dict.items():
...
Dictionary Comprehension¶
Like lists and sets, dictionaries can be constructed using comprehension. Unlike lists and sets, however, dictionary comprehension looks like:
my_dict = {key: value for ...}
Usage with **kwargs¶
If you want to pass the contents of a dictionary into a function’s **kwargs,
you can do the following:
my_function(**my_dict)
If my_dict contains key-value pairs key1: value1 and key2: value2, this
is equivalent to:
my_function(key1=value1, key2=value2)
Remember!
Using **kwargs in the parameter list of a function creates a local
variable kwargs that is a dictionary containing all additional positional
inputs.
Dictionary Operations¶
See Mapping Types.
Important to Remember
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