Dictionaries

TODO: CLEANUP

# Dictionaries store mappings from keys to values
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}
print(filled_dict)

Note: keys for dictionaries have to be immutable types. This is to ensure that the key can be converted to a constant hash value for quick look-ups.

Immutable types include ints, floats, strings, tuples.

invalid_dict = {[1,2,3]: "123"}  # => Yield a TypeError: unhashable type: 'list'
valid_dict = {(1,2,3):[1,2,3]}   # Values can be of any type, however.

filled_dict = {"one": 1, "two": 2, "three": 3}
# Look up values with []
print(filled_dict["one"])  # => 1

Get all keys as an iterable with keys(). We need to wrap the call in list() to turn it into a list. We'll talk about those later.

Note: for Python versions <3.7, dictionary key ordering is not guaranteed. Your results might not match the example below exactly. However, as of Python 3.7, dictionary items maintain the order at which they are inserted into the dictionary.

filled_dict = {"one": 1, "two": 2, "three": 3}
print(list(filled_dict.keys()))  # => ["three", "two", "one"] in Python <3.7
                                 # => ["one", "two", "three"] in Python 3.7+

Get all values as an iterable with values(). Once again we need to wrap it in list() to get it out of the iterable.

Note: Same as above regarding key ordering.

filled_dict = {"one": 1, "two": 2, "three": 3}
print(list(filled_dict.values()))  # => [3, 2, 1]  in Python <3.7
                                   # => [1, 2, 3] in Python 3.7+

Check for existence of keys in a dictionary with in

filled_dict = {"one": 1, "two": 2, "three": 3}
print("one" in filled_dict)  # => True
print(1 in filled_dict)      # => False

Looking up a non-existing key is a KeyError

filled_dict = {"one": 1, "two": 2, "three": 3}
print(filled_dict["four"])  # KeyError

Use get() method to avoid the KeyError

filled_dict = {"one": 1, "two": 2, "three": 3}
print(filled_dict.get("one"))      # => 1
print(filled_dict.get("four"))     # => None

# The get method supports a default argument when the value is missing
print(filled_dict.get("one", 4))   # => 1
print(filled_dict.get("four", 4))  # => 4

setdefault() inserts into a dictionary only if the given key isn't present

filled_dict = {"one": 1, "two": 2, "three": 3}

filled_dict.setdefault("five", 5)  # filled_dict["five"] is set to 5
print(filled_dict["five"])

filled_dict.setdefault("five", 6)  # filled_dict["five"] is still 5
print(filled_dict["five"])

Adding to a dictionary

filled_dict = {"one": 1, "two": 2, "three": 3, "five": 5}
print(filled_dict)

filled_dict.update({"four":4})  # => {"one": 1, "two": 2, "three": 3, "four": 4}
print(filled_dict)

filled_dict["four"] = 4         # another way to add to dict
print(filled_dict)

filled_dict = {"one": 1, "two": 2, "three": 3}

# Remove keys from a dictionary with del
del filled_dict["one"]  # Removes the key "one" from filled dict
print(filled_dict)

# From Python 3.5 you can also use the additional unpacking options
print({"a": 1, **{"b": 2}})  # => {'a': 1, 'b': 2}
print({"a": 1, **{"a": 2}})  # => {'a': 2}

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