Python Dictionaries Basics

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Python Dictionaries

Python Dictionaries are

• unordered collections of arbitrary objects
• built-in mutable mapping that maps keys to values

in dictionaries, items are stored and fetched by key.

'''Basic Dictionary Operations'''

D = {'spam': 2, 'ham': 1, 'eggs': 3}                   # dict in literal expression
# key indexing support fetch, add, change, delete
print(D['spam'])                                       # fetch a value by key
D['brunch'] = 'Bacon'                                  # add new entry
D['ham'] = ['grill', 'bake', 'fry']                    # change entry
del D['egg']                                           # delete entry
print(D)

'''
2
{'brunch': 'Bacon', 'spam': 2, 'ham': ['grill', 'bake', 'fry']}
'''

'''Dictionary Methods'''
D = {'spam': 2, 'ham': 1, 'eggs': 3}

# d.keys(), d.values(), d.items()
# useful in loops that need to step through dictionary entries one by one
for element in [D.keys(),
                D.values(),
                D.items()]:                           # (key,value) pair tuples
    print(list(element))

# d.get(key[, default])
print(
        D.get('spam'),
        D.get('toast'),                               # a key that is missing
        D.get('toast', 88),
        '\n',
        end=""
)

# d.update([other])
D2 = {'toast':4, 'muffin':5}                          # would overwrite values of the same key if there’s a clash
D.update(D2)
print(D)

# d.pop(key[, default])
# deletes a key from a dictionary and returns the value it had
print(
        D.pop('muffin'),
        D.pop('toast'),
        D
)


'''
['spam', 'ham', 'eggs']
[2, 1, 3]
[('spam', 2), ('ham', 1), ('eggs', 3)]
2 None 88
{'spam': 2, 'ham': 1, 'eggs': 3, 'toast': 4, 'muffin': 5}
5 4 {'spam': 2, 'ham': 1, 'eggs': 3}
'''

'''Other Ways to Make Dictionaries'''
# -------two basic forms------- #
a = {'one': 1, 'two': 2, 'three': 3}                    # literal expression

b = {}                                                  # assign by keys dynamically
b['one'] = 1
b['two'] = 2
b['three'] = 3

# -------dict constructor------- #
# dict(**kwarg)
c = dict(one=1, two=2, three=3)                         # requires all keys to be strings

# dict(mapping, **kwarg)
d = dict({'three': 3, 'one': 1, 'two': 2})

# dict(iterable, **kwarg)
e = dict([('two', 2), ('one', 1)], three = 3)

keyslist = ['one', 'two', 'three']
valueslist = [1, 2, 3]
zipObject = zip(keyslist, valueslist)                   # first use zip() to construct iterable
# print(list(zipObject))
f = dict(zipObject)

# -------dictionary comprehension expression------- #
g = {k:v for (k, v) in zip(keyslist, valueslist)}       # iterable object can only be iterated once

print(a==b==c==d==e==f==g)

'''
True
'''

Python Dictionaries Basics

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原文地址：https://www.cnblogs.com/bit-happens/p/11869860.html