Python语法精髓

# # Python语法精髓

以下所有实例出自python3官方文档：https://docs.python.org/3/tutorial/index.html

# # List

list是一种有序的数据集合

    squares = [1, 4, 9, 16, 25]
    
    # 取值
    squares[0]  # indexing returns the item // 1
    squares[-1] # 25
    
    # 拷贝新数组
    squares[-3:]  # slicing returns a new list // [9, 16, 25]
    squares[:] # returns a shallow copy of the list: // [1, 4, 9, 16, 25]
    
    # 连接
    squares + [36, 49, 64, 81, 100] # [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
    
    squares[0] = 2 # 修改值
    squares.append(5)
    len(squares)
    for fruit in squares:
    # 数组特性
    squares.pop()/append(var)/reverse()/sort()/insert(index, var)
    # 其他
    squares.remove(var)/count(var)/index(var)
    
    letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
    # replace some values
    letters[2:5] = ['C', 'D', 'E']
    # now remove them
    letters[2:5] = []
    # clear the list by replacing all the elements with an empty list
    letters[:] = []

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# # if表达式

if statement:

    if condition: statement
    if condition:
        block

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if expression (introduced in Python 2.5)

    # 类似js三元表达式的变种
    expression_if_true if condition else expression_if_false

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    x = a if b else 0 // 等价于 x = (a if b else 0)，等价于js：x = b ? a : 0

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# # 返回多参数（元祖）

    # When writing tuples, the parenthesis can be omitted most of the times.
    # point = 2, 3
    def foo (a):
        x=a
        y=a*2
        return x,y # 作为元祖返回，可带括号或不带
    
    x,y = foo(50)

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# # 字符串格式化

Python uses C-style string formatting to create new, formatted strings. The "%" operator is used to format a set of variables enclosed in a "tuple" (a fixed size list), together with a format string, which contains normal text together with "argument specifiers", special symbols like "%s" and "%d".

    name = "John"
    age = 23
    print("%s is %d years old." % (name, age))

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# # 多行语句

可以使用斜杠（ \）将一行的语句分为多行显示.

    total = item_one + \
            item_two + \
            item_three

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字符串支持多行显示，使用三个引号"""..."""或'''...'''

    print("""
    Usage: thingy [OPTIONS]
         -h                        Display this usage message
         -H hostname               Hostname to connect to
    """)

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字符串可以使用引号( ' )、双引号( " )、三引号( ''' 或 """ )。三引号可以由多行组成，常用于文档字符串。

# # for ... in

for ... in适用于list/dict/set。

dict:

items():返回元祖list
keys():返回key list
values():返回value list

    >>> for item in {'color': 'blue', 'fruit': 'apple'}:
    ...     print(item)
    ...
    ('color', 'blue')
    ('fruit', 'apple')
    
    # >>> 等价for key, value in a_dict.items():

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# # 推导式

推导式comprehensions（又称解析式），是Python的一种独有特性。推导式是可以从一个数据序列构建另一个新的数据序列的结构体。共有三种推导式，在Python2和3中都有支持：

列表(list)推导式
字典(dict)推导式
集合(set)推导式

# # 1. list

基本格式：variable = [out_exp_res for out_exp in input_list if out_exp == 2]

    # map:
    >>> squares = [x**2 for x in range(10)]
    # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
    
    # filter
    [x for x in vec if x >= 0]
    
    # 复杂表达式
    >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
    # [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
    # 等价于
    >>> combs = []
    >>> for x in [1,2,3]:
    ...     for y in [3,1,4]:
    ...         if x != y:
    ...             combs.append((x, y))
    ...
    >>> combs
    [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

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# # 2. dict

基本格式：{ key_expr: value_expr for value in collection if condition }

    >>> objects = ['blue', 'apple', 'dog']
    >>> categories = ['color', 'fruit', 'pet']
    >>> a_dict = {key: value for key, value in zip(categories, objects)}
    >>> a_dict
    {'color': 'blue', 'fruit': 'apple', 'pet': 'dog'}
    
    # Turning Keys Into Values
    >>> a_dict = {'one': 1, 'two': 2, 'thee': 3, 'four': 4}
    >>> new_dict = {value: key for key, value in a_dict.items()}
    >>> new_dict
    {1: 'one', 2: 'two', 3: 'thee', 4: 'four'}
    
    # filter
    >>> a_dict = {'one': 1, 'two': 2, 'thee': 3, 'four': 4}
    >>> new_dict = {k: v for k, v in a_dict.items() if v <= 2}
    >>> new_dict
    {'one': 1, 'two': 2}
    
    # 以下等价于sum(incomes.values())
    >>> incomes = {'apple': 5600.00, 'orange': 3500.00, 'banana': 5000.00}
    >>> total_income = sum([value for value in incomes.values()])
    >>> total_income
    14100.0

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# # 3. set

基本格式：{ expr for value in collection if condition }

    squared = {x**2 for x in [1, 1, 2]}
    # Output: set([1, 4])

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# # 4. 区别

list和set output都是key表达式，但list使用[],set使用{}
dict和set 都是使用{}，但dict output是key:value表达式，set output是key表达式

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