dicts.py

# dicts.py


>>> a = dict(A=1, Z=-1)
>>> b = {'A': 1, 'Z': -1}
>>> c = dict(zip(['A', 'Z'], [1, -1]))
>>> d = dict([('A', 1), ('Z', -1)])
>>> e = dict({'Z': -1, 'A': 1})
>>> a == b == c == d == e  # are they all the same?
True  # They are indeed


# zip
>>> list(zip(['h', 'e', 'l', 'l', 'o'], [1, 2, 3, 4, 5]))
[('h', 1), ('e', 2), ('l', 3), ('l', 4), ('o', 5)]
>>> list(zip('hello', range(1, 6)))  # equivalent, more pythonic
[('h', 1), ('e', 2), ('l', 3), ('l', 4), ('o', 5)]


# basic
>>> d = {}
>>> d['a'] = 1  # let's set a couple of (key, value) pairs
>>> d['b'] = 2
>>> len(d)  # how many pairs?
2
>>> d['a']  # what is the value of 'a'?
1
>>> d  # how does `d` look now?
{'a': 1, 'b': 2}
>>> del d['a']  # let's remove `a`
>>> d
{'b': 2}
>>> d['c'] = 3  # let's add 'c': 3
>>> 'c' in d  # membership is checked against the keys
True
>>> 3 in d  # not the values
False
>>> 'e' in d
False
>>> d.clear()  # let's clean everything from this dictionary
>>> d
{}



# views
>>> d = dict(zip('hello', range(5)))
>>> d
{'h': 0, 'e': 1, 'l': 3, 'o': 4}
>>> d.keys()
dict_keys(['h', 'e', 'l', 'o'])
>>> d.values()
dict_values([0, 1, 3, 4])
>>> d.items()
dict_items([('h', 0), ('e', 1), ('l', 3), ('o', 4)])
>>> 3 in d.values()
True
>>> ('o', 4) in d.items()
True


# other methods
>>> d
{'e': 1, 'h': 0, 'o': 4, 'l': 3}
>>> d.popitem()  # removes a random item (useful in algorithms)
('o', 4)
>>> d
{'h': 0, 'e': 1, 'l': 3}
>>> d.pop('l')  # remove item with key `l`
3
>>> d.pop('not-a-key')  # remove a key not in dictionary: KeyError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'not-a-key'
>>> d.pop('not-a-key', 'default-value')  # with a default value?
'default-value'  # we get the default value
>>> d.update({'another': 'value'})  # we can update dict this way
>>> d.update(a=13)  # or this way (like a function call)
>>> d
{'h': 0, 'e': 1, 'another': 'value', 'a': 13}
>>> d.get('a')  # same as d['a'] but if key is missing no KeyError
13
>>> d.get('a', 177)  # default value used if key is missing
13
>>> d.get('b', 177)  # like in this case
177
>>> d.get('b')  # key is not there, so None is returned


# setdefault
>>> d = {}
>>> d.setdefault('a', 1)  # 'a' is missing, we get default value
1
>>> d
{'a': 1}  # also, the key/value pair ('a', 1) has now been added
>>> d.setdefault('a', 5)  # let's try to override the value
1
>>> d
{'a': 1}  # no override, as expected


# setdefault example
>>> d = {}
>>> d.setdefault('a', {}).setdefault('b', []).append(1)
>>> d
{'a': {'b': [1]}}