python 快速入门python简介

Python 是一个高层次的结合了解释性、编译性、互动性和面向对象的脚本语言。

Python 是一种解释型语言： 这意味着开发过程中没有了编译这个环节。类似于PHP和Perl语言。Python 是交互式语言： 这意味着，您可以在一个 Python 提示符 >>> 后直接执行代码。Python 是面向对象语言: 这意味着Python支持面向对象的风格或代码封装在对象的编程技术。python版本

python目前支持两个大的版本Python, 2.7 and 3.5. 令人疑惑的是python的2和3的版本不兼容，因此使用2的版本的代码在3中可能会出现不兼容的现象，目前推荐使用python 3+的版本作为开放和学习。

基本的数据类型

像大多数语言一样，Python具有许多基本类型，包括整数，浮点数，布尔值和字符串。这些数据类型以其他编程语言所熟悉的方式运行。

数值：

整数和浮点数可以像其他语言一样使用：

x = 3print(type(x)) # Prints "<class 'int'>"print(x)       # Prints "3"print(x + 1)   # Addition; prints "4"print(x - 1)   # Subtraction; prints "2"print(x * 2)   # Multiplication; prints "6"print(x ** 2)  # Exponentiation; prints "9"x += 1print(x)  # Prints "4"x *= 2print(x)  # Prints "8"y = 2.5print(type(y)) # Prints "<class 'float'>"print(y, y + 1, y * 2, y ** 2) # Prints "2.5 3.5 5.0 6.25"

python不像其他编程语言，其不支持x++和x—这种操作。

布尔值：

不同于其他编程语言，python使用英文代替(||, &&等):

t = Truef = Falseprint(type(t)) # Prints "<class 'bool'>"print(t and f) # Logical AND; prints "False"   与print(t or f)  # Logical OR; prints "True"     或print(not t)   # Logical NOT; prints "False"   非print(t != f)  # Logical XOR; prints "True"

字符串

python对字符串的支持是很强大的:

hello = 'hello'    # 单引号world = "world"    # 双引号也可print(hello)       # Prints "hello"print(len(hello))  # String length; prints "5"hw = hello + ' ' + world  # String concatenationprint(hw)  # prints "hello world"hw12 = '%s %s %d' % (hello, world, 12)  # sprintf style string formattingprint(hw12)  # prints "hello world 12"

Python包含几种内置的容器类型: lists, dictionaries, sets和tuples.

Lists

列表与数组的Python等效，但可调整大小，并且可以包含不同类型的元素：

xs = [3, 1, 2]    # Create a listprint(xs, xs[2])  # Prints "[3, 1, 2] 2"print(xs[-1])     # Negative indices count from the end of the list; prints "2"xs[2] = 'foo'     # Lists can contain elements of different typesprint(xs)         # Prints "[3, 1, 'foo']"xs.append('bar')  # Add a new element to the end of the listprint(xs)         # Prints "[3, 1, 'foo', 'bar']"x = xs.pop()      # Remove and return the last element of the listprint(x, xs)      # Prints "bar [3, 1, 'foo']"

切片: 除了一次访问一个列表元素，Python还提供了简洁的语法来访问子列表。这称为切片

nums = list(range(5))     # range is a built-in function that creates a list of integersprint(nums)               # Prints "[0, 1, 2, 3, 4]"print(nums[2:4])          # Get a slice from index 2 to 4 (exclusive); prints "[2, 3]"print(nums[2:])           # Get a slice from index 2 to the end; prints "[2, 3, 4]"print(nums[:2])           # Get a slice from the start to index 2 (exclusive); prints "[0, 1]"print(nums[:])            # Get a slice of the whole list; prints "[0, 1, 2, 3, 4]"print(nums[:-1])          # Slice indices can be negative; prints "[0, 1, 2, 3]"nums[2:4] = [8, 9]        # Assign a new sublist to a sliceprint(nums)               # Prints "[0, 1, 8, 9, 4]"

循环:

animals = ['cat', 'dog', 'monkey']for animal in animals:    print(animal)

如果要访问循环体内每个元素的索引，请使用内置的枚举函数enumerate：

animals = ['cat', 'dog', 'monkey']for idx, animal in enumerate(animals):    print('#%d: %s' % (idx + 1, animal))

列表推导式(List comprehensions):

列表推导式可以将循环这种方式表达的更为简洁，如下所示:

# 构建1-10的列表l1 = [x for x in range(1,11)]# 可以对迭代的元素进行操作l2= [x*x for x in range(1,11)]# for循环后跟if语句l3 = [i for i in range(1,11) if i % 2 == 0]# 嵌套列表合成单个列表 [[1,2],[3,4]] -> [1,2,3,4]l = [[1,2],[3,4]]l4 = [j for i in l for j in i]

字典是是无序的键值对（key:value）集合，等同于c++中的unordered_map哈希表。同一个字典内的键必须是互不相同的。

其形式为 :键：值

# 创建字典d = {'cat': 'cute', 'dog': 'furry'}  # 获取key值print(d['cat'])       # Get an entry from a # 查询key是不是在字典中print('cat' in d)     # 新增key:valued['fish'] = 'wet'     # Set an entry in a dictionaryprint(d['fish'])      # Prints "wet"# 如果没有key，则报错，所以一般不直接进行取值操作# print(d['monkey'])  # KeyError: 'monkey' not a key of d# 使用get获取key值，如果key不存在，则赋予默认值print(d.get('monkey', 'N/A')) # 删除元素del d['fish']

迭代

迭代字典很简单，和列表差不多:

d = {'person': 2, 'cat': 4, 'spider': 8}# 注意这种遍历的是key的值for animal in d:          #key    legs = d[animal]      #value    print 'A %s has %d legs' % (animal, legs)# Prints "A person has 2 legs", "A spider has 8 legs", "A cat has 4 legs"# 如果想直接获得键值对, 使用iteritems:d = {'person': 2, 'cat': 4, 'spider': 8}for animal, legs in d.iteritems():    print 'A %s has %d legs' % (animal, legs)# Prints "A person has 2 legs", "A spider has 8 legs", "A cat has 4 legs"

字典的表达式

nums = [0, 1, 2, 3, 4]even_num_to_square = {x: x ** 2 for x in nums if x % 2 == 0}print even_num_to_square  # Prints "{0: 0, 2: 4, 4: 16}"

集合，也就是没有顺序的，同时所有的元素都不相同的集合。A set is an unordered collection of distinct elements，用英文更好的表达其含义。集合同样是用花括号创建：

animals = {'cat', 'dog'}print('cat' in animals)   # Check if an element is in a set; prints "True"print('fish' in animals)  # prints "False"animals.add('fish')       # Add an element to a setprint('fish' in animals)  # Prints "True"print(len(animals))       # Number of elements in a set; prints "3"animals.add('cat')        # Adding an element that is already in the set does nothingprint(len(animals))       # Prints "3"animals.remove('cat')     # Remove an element from a setprint(len(animals))       # Prints "2"

集合的循环很列表一致：

animals = {'cat', 'dog', 'fish'}for idx, animal in enumerate(animals):    print '#%d: %s' % (idx + 1, animal)# Prints "#1: fish", "#2: dog", "#3: cat"

元组是（不可变的）有序值列表。元组在很多方面类似于列表。最重要的区别之一是，元组可以用作字典中的键和集合的元素，而列表则不能。这是一个简单的示例：

# 使用括号t = (5, 6)        # Create a tupleprint(type(t))    # Prints "<class 'tuple'>"

python函数的定义使用def:

def sign(x):    if x > 0:        return 'positive'    elif x < 0:        return 'negative'    else:        return 'zero'for x in [-1, 0, 1]:    print(sign(x))# Prints "negative", "zero", "positive"

类的定义很简单，如下所示的基本结构:

class Greeter(object):    # 构造函数    def __init__(self, name):        self.name = name  # Create an instance variable    # 方法    def greet(self, loud=False):        if loud:            print('HELLO, %s!' % self.name.upper())        else:            print('Hello, %s' % self.name)g = Greeter('Fred')  # Construct an instance of the Greeter classg.greet()            # Call an instance method; prints "Hello, Fred"g.greet(loud=True)   # Call an instance method; prints "HELLO, FRED!"

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