第八章 List Dictionaries

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1.List

>>> len([1, 2, 3]) # Length
3
>>> [1, 2, 3] + [4, 5, 6] # Concatenation
[1, 2, 3, 4, 5, 6]
>>> ['Ni!'] * 4 # Repetition
['Ni!', 'Ni!', 'Ni!', 'Ni!']

>>> 3 in [1, 2, 3] # Membership
True
>>> for x in [1, 2, 3]:
... print(x, end=' ') # Iteration (2.X uses: print x,)
...
1 2 3


>>> res = [c * 4 for c in 'SPAM'] # List comprehensions
>>> res
['SSSS', 'PPPP', 'AAAA', 'MMMM']

>>> res = []
>>> for c in 'SPAM': # List comprehension equivalent
... res.append(c * 4)
...
>>> res
['SSSS', 'PPPP', 'AAAA', 'MMMM']


>>> L = ['spam', 'Spam', 'SPAM!']
>>> L[2] # Offsets start at zero
'SPAM!'
>>> L[−2] # Negative: count from the right
'Spam'
>>> L[1:] # Slicing fetches sections
['Spam', 'SPAM!']


>>> matrix = [[1, 2, 3],
... [4, 5, 6],
... [7, 8, 9]]
>>> matrix[1][1]
5

>>> L = ['spam', 'Spam', 'SPAM!']
>>> L[1] = 'eggs' # Index assignment
>>> L
['spam', 'eggs', 'SPAM!']
>>> L[0:2] = ['eat', 'more'] # Slice assignment: delete+insert    解释可以赋值超过原来个数
>>> L # Replaces items 0,1
['eat', 'more', 'SPAM!']


>>> L = [1, 2, 3]
>>> L[1:2] = [4, 5] # Replacement/insertion
>>> L
[1, 4, 5, 3]
>>> L[1:1] = [6, 7] # Insertion (replace nothing)
>>> L
[1, 6, 7, 4, 5, 3]
>>> L[1:2] = [] # Deletion (insert nothing)
>>> L
[1, 7, 4, 5, 3]

>>> L = ['eat', 'more', 'SPAM!']
>>> L.append('please') # Append method call: add item at end
>>> L
['eat', 'more', 'SPAM!', 'please']
>>> L.sort() # Sort list items ('S' < 'e')
>>> L
['SPAM!', 'eat', 'more', 'please']

>>> L = ['abc', 'ABD', 'aBe']
>>> L.sort() # Sort with mixed case
>>> L
['ABD', 'aBe', 'abc']
>>> L = ['abc', 'ABD', 'aBe']
>>> L.sort(key=str.lower) # Normalize to lowercase
>>> L
['abc', 'ABD', 'aBe']
>>>
>>> L = ['abc', 'ABD', 'aBe']
>>> L.sort(key=str.lower, reverse=True) # Change sort order
>>> L
['aBe', 'ABD', 'abc']


>>> L = ['abc', 'ABD', 'aBe']
>>> sorted(L, key=str.lower, reverse=True) # Sorting built-in
['aBe', 'ABD', 'abc']
>>> L = ['abc', 'ABD', 'aBe']
>>> sorted([x.lower() for x in L], reverse=True) # Pretransform items: differs!
['abe', 'abd', 'abc']

>>> L = [1, 2]
>>> L.extend([3, 4, 5]) # Add many items at end (like in-place +)
>>> L
[1, 2, 3, 4, 5]
>>> L.pop() # Delete and return last item (by default: −1)
5
>>> L
[1, 2, 3, 4]
>>> L.reverse() # In-place reversal method
>>> L
[4, 3, 2, 1]
>>> list(reversed(L)) # Reversal built-in with a result (iterator)
[1, 2, 3, 4]

>>> L = ['spam', 'eggs', 'ham', 'toast']
>>> del L[0] # Delete one item
>>> L
['eggs', 'ham', 'toast']
>>> del L[1:] # Delete an entire section
>>> L # Same as L[1:] = []
['eggs']

2.Dictionaries

>>> D
{'eggs': 3, 'spam': 2, 'ham': 1}
>>> D['ham'] = ['grill', 'bake', 'fry'] # Change entry (value=list)
>>> D
{'eggs': 3, 'spam': 2, 'ham': ['grill', 'bake', 'fry']}
>>> del D['eggs'] # Delete entry
>>> D
{'spam': 2, 'ham': ['grill', 'bake', 'fry']}
>>> D['brunch'] = 'Bacon' # Add new entry
>>> D
{'brunch': 'Bacon', 'spam': 2, 'ham': ['grill', 'bake', 'fry']}

>>> D = {'spam': 2, 'ham': 1, 'eggs': 3}
>>> list(D.values())
[3, 2, 1]
>>> list(D.items())
[('eggs', 3), ('spam', 2), ('ham', 1)]
>>> D.get('spam') # A key that is there
2
>>> print(D.get('toast')) # A key that is missing
None
>>> D.get('toast', 88)
88
>>> D
{'eggs': 3, 'spam': 2, 'ham': 1}
>>> D2 = {'toast':4, 'muffin':5} # Lots of delicious scrambled order here
>>> D.update(D2)
>>> D
{'eggs': 3, 'muffin': 5, 'toast': 4, 'spam': 2, 'ham': 1}

# pop a dictionary by key
>>> D
{'eggs': 3, 'muffin': 5, 'toast': 4, 'spam': 2, 'ham': 1}
>>> D.pop('muffin')
5
>>> D.pop('toast') # Delete and return from a key
4
>>> D
{'eggs': 3, 'spam': 2, 'ham': 1}
# pop a list by position
>>> L = ['aa', 'bb', 'cc', 'dd']
>>> L.pop() # Delete and return from the end
'dd
>>> L
['aa', 'bb', 'cc']
>>> L.pop(1) # Delete from a specific position
'bb'
>>> L
['aa', 'cc']

>>> table = {'Holy Grail': '1975', # Key=>Value (title=>year)
... 'Life of Brian': '1979',
... 'The Meaning of Life': '1983'}
>>>
>>> table['Holy Grail']
'1975'
>>> list(table.items()) # Value=>Key (year=>title)
[('The Meaning of Life', '1983'), ('Holy Grail', '1975'), ('Life of Brian', '1979')]
>>> [title for (title, year) in table.items() if year == '1975']
['Holy Grail']

>>> Matrix={(2, 3, 4): 88, (7, 8, 9): 99}
>>> if (2, 3, 6) in Matrix: # Check for key before fetch
...   print(Matrix[(2, 3, 6)]) # See Chapters 10 and 12 for if/else
... else:
...   print(0)
...
0
>>> try:
... print(Matrix[(2, 3, 6)]) # Try to index
... except KeyError: # Catch and recover
... print(0) # See Chapters 10 and 34 for try/except
...
0
>>> Matrix.get((2, 3, 4), 0) # Exists: fetch and return
88
>>> Matrix.get((2, 3, 6), 0) # Doesn't exist: use default arg
0

{'name': 'Bob', 'age': 40} # Traditional literal expression
D = {} # Assign by keys dynamically
D['name'] = 'Bob'
D['age'] = 40
dict(name='Bob', age=40) # dict keyword argument form
dict([('name', 'Bob'), ('age', 40)]) # dict key/value tuples form
dict(zip(keyslist, valueslist)) # Zipped key/value tuples form (ahead)

>>> dict.fromkeys(['a', 'b'], 0)
{'a': 0, 'b': 0}

>>> list(zip(['a', 'b', 'c'], [1, 2, 3])) # Zip together keys and values
[('a', 1), ('b', 2), ('c', 3)]
>>> D = dict(zip(['a', 'b', 'c'], [1, 2, 3])) # Make a dict from zip result
>>> D
{'b': 2, 'c': 3, 'a': 1}

>>> D = {k: v for (k, v) in zip(['a', 'b', 'c'], [1, 2, 3])}
>>> D
{'b': 2, 'c': 3, 'a': 1}



>>> D = dict.fromkeys(['a', 'b', 'c'], 0) # Initialize dict from keys
>>> D
{'b': 0, 'c': 0, 'a': 0}
>>> D = {k:0 for k in ['a', 'b', 'c']} # Same, but with a comprehension
>>> D
{'b': 0, 'c': 0, 'a': 0}

>>> D = dict(a=1, b=2, c=3)
>>> D
{'b': 2, 'c': 3, 'a': 1}
>>> K = D.keys() # Makes a view object in 3.X, not a list
>>> K
dict_keys(['b', 'c', 'a'])
>>> list(K) # Force a real list in 3.X if needed
['b', 'c', 'a']
>>> V = D.values() # Ditto for values and items views
>>> V
dict_values([2, 3, 1])
>>> list(V)
[2, 3, 1]
>>> D.items()
dict_items([('b', 2), ('c', 3), ('a', 1)])
>>> list(D.items())
[('b', 2), ('c', 3), ('a', 1)]
>>> K[0] # List operations fail unless converted
TypeError: 'dict_keys' object does not support indexing
>>> list(K)[0]
'b'



>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> D
{'b': 2, 'c': 3, 'a': 1}
>>> K = D.keys()
>>> V = D.values()
>>> list(K) # Views maintain same order as dictionary
['b', 'c', 'a']
>>> list(V)
[2, 3, 1]
>>> del D['b'] # Change the dictionary in place
>>> D
{'c': 3, 'a': 1}
>>> list(K) # Reflected in any current view objects
['c', 'a']
>>> list(V) # Not true in 2.X! - lists detached from dict
[3, 1]

>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> D.keys() & D.keys() # Intersect keys views
{'b', 'c', 'a'}
>>> D.keys() & {'b'} # Intersect keys and set
{'b'}
>>> D.keys() & {'b': 1} # Intersect keys and dict
{'b'}
>>> D.keys() | {'b', 'c', 'd'} # Union keys and set
{'b', 'c', 'a', 'd'}

>>> D = {'a': 1}
>>> list(D.items()) # Items set-like if hashable
[('a', 1)]
>>> D.items() | D.keys() # Union view and view
{('a', 1), 'a'}
>>> D.items() | D # dict treated same as its keys
{('a', 1), 'a'}
>>> D.items() | {('c', 3), ('d', 4)} # Set of key/value pairs
{('d', 4), ('a', 1), ('c', 3)}
>>> dict(D.items() | {('c', 3), ('d', 4)}) # dict accepts iterable sets too
{'c': 3, 'a': 1, 'd': 4}

>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> D
{'b': 2, 'c': 3, 'a': 1}
>>> Ks = D.keys() # Sorting a view object doesn't work!
>>> Ks.sort()
AttributeError: 'dict_keys' object has no attribute 'sort'


>>> Ks = list(Ks) # Force it to be a list and then sort
>>> Ks.sort()
>>> for k in Ks: print(k, D[k]) # 2.X: omit outer parens in prints
...
a 1
b 2
c 3
>>> D
{'b': 2, 'c': 3, 'a': 1}
>>> Ks = D.keys() # Or you can use sorted() on the keys
>>> for k in sorted(Ks): print(k, D[k]) # sorted() accepts any iterable
... # sorted() returns its result
a 1
b 2
c 3

>>> D
{'b': 2, 'c': 3, 'a': 1} # Better yet, sort the dict directly
>>> for k in sorted(D): print(k, D[k]) # dict iterators return keys
...
a 1
b 2
c 3