【笔记】《Python语言以及应用》- 基础语法

2018-04-01  本文已影响13人  u14e

主要内容:

  1. Python基本元素:数字、字符串和变量
  2. Python容器:列表、元组、字典、集合
  3. 程序结构

一、Python基本元素:数字、字符串和变量


Python基本数据类型(不可变): bool, int, float, str

1. 变量

变量名只能包含大小写字母、数字、下划线,开头不能为数字

type()检查对象的类型: type('hello') # <class 'str'>

2. 数字

常见运算: +, -, *, /(浮点数除法,结果为小数), //(整除,结果为整数), %(求余), **(幂)

自运算: -=, +=, *=, /=, ...

类型转换: int(), float()

隐式转换: int + float = float, bool + int = int

divmod(9, 5) # (1, 4) 类似于(9 // 5, 9 % 5),获得整数和余数

3. 字符串

三元引号用于创建多行字符串: 每一行的换行符以及行首和行尾的空格都会被保留

single = 'hello world'
multi = '''
  hello
  world
'''
print(len(single))  # 11
print(len(multi))   # 17

类型转换: str()

转义字符: \n, \t, \', \", \\, \000(空格)

常用操作:

字符串常用方法:

二、Python容器:列表、元组、字典、集合


1. 列表(list: 可变、有序、重复)

创建列表: list()或者[]

转换:

使用[offset]获取和修改元素((超过偏移量会报错))

使用切片[start:end:step]提取部分列表(同字符串切片)

操作数组:

数组复制:

2. 元组(tuple: 不可变)

元组占用空间小
不会意外修改元组的值
可以用作字典的键
命名元组可以替代对象
函数的参数是以元组的形式传递的

# 创建元组
empty_tuple = ()
one_char = 'a', 
char_tuple = 'a', 'b', 'c'
character = ('a', 'b', 'c')

# 元组解包
a, b, c = character

# 互换
b, a = a, b

# 转换
tuple(list)

3. 字典(dict: 可变)

创建字典: {}

转换:dict()(将包含双值子序列的序列转换为字典)

lol = [['a', 1], ['b', 2], ['c', 3]]  # (a list of two-item list)
lot = [('a', 1), ('b', 2), ('c', 3)]  # (a list of two-item tuple)
tol = (['a', 1], ['b', 2], ['c', 3])  # (a tuple of two-item list)
los = ['a1', 'b2', 'c3']
tos = ('a1', 'b2', 'c3')

dict(lol)   # {'a': 1, 'b': 2, 'c': 3}
dict(lot)   # {'a': 1, 'b': 2, 'c': 3} 
dict(tol)   # {'a': 1, 'b': 2, 'c': 3} 
dict(los)   # {'a': '1', 'b': '2', 'c': '3'}
dict(tos)   # {'a': '1', 'b': '2', 'c': '3'}

常用操作

4. 集合(set: 无序、不重复、可变)

集合就像舍弃了值, 仅剩下键的字典

创建集合: set(), {1, 3, 6, 9}

转换:

set('letter')    # {'l', 'r', 't', 'e'}
set(['a', 'b', 'c'])      # {'c', 'b', 'a'}
set(('a', 'b', 'c'))      # {'c', 'b', 'a'}
set({'a': 1, 'b': 2, 'c': 3})    # {'c', 'b', 'a'}

in检查是否存在

运算符:

a = {1, 2}
b = {2, 3}

& 交集(a.intersection(b))
a & b # {2}

| 并集(a.union(b))
a | b # {1, 2, 3}

- 差集(a.difference(b))(出现在第一个集合但不出现在第二个集合)
a - b # {1}

^ 异或集(a.symmetric_difference(b))(两个集合中的不同元素)
a ^ b # {1, 3}

<= 判断子集(a.issubset(b))
a <= b  # False

>= 判断真集(a.issuperset(b))
a >= b  # False

三、程序结构


1. #注释

Python没有多行注释

2. \连接

把一段长代码分成多行()

# 拼接长字符串
alphabet = ''
alphabet += 'abc'
alphabet += 'def'

# 使用\
alphabet = 'abc' + \
          'def'

1 + 2 + 3
使用\
1 + 2 + \
3

3. if elif else条件判断

num = 2
if num == 1:
  print('This is one')
elif num === 2:
  print('This is two')
else:
  print('I don\'t known')

判断假值

类型 假值
bool False
null类型 None
int 0
float 0.0
str ''
list []
tuple ()
dict {}
set set()

4. while循环

# 读入整数,如果是奇数则输出其平方数,偶数则跳过,q来结束循环
while True:
  value = input('Integer, please [q to quit]: ')

  if value == 'q':
    break             # break 跳出循环
  
  number = int(value)

  if number % 2 == 0:
    continue          # 跳出本次循环,继续下一个循环
  
  print(number, 'squared is', number**2)

循环外使用else: 如果while循环正常结束(没有使用break跳出),程序将执行可选的else片段

5. for...in迭代

list, str, tuple, dict, set都是可迭代的对象
break, continue, else和while一样

range(start, stop, step)生成自然数序列(不包括stop)

zip()并行迭代

alphabet = ['a', 'b', 'c']
number = [1, 2, 3, 4]
for c, n in zip(alphabet, number):
  print(c, n)

# 在最短序列用完时停止
# a 1
# b 2
# c 3

# 配对两个元组
list(zip(alphabet, number)) # [('a', 1), ('b', 2), ('c', 3)]

# 字典
dict(zip(alphabet, number)) # {'c': 3, 'a': 1, 'b': 2}

6. 推导式

列表推导式:

# [expression for item in iterable]
number_list = [x for x in range(3)]  # [0, 1, 2]

# [expression for item in iterable if condition]
a_list = [x for x in range(10) if x % 2 == 0] # [0, 2, 4, 6, 8]

# 嵌套循环
rows = range(1, 4)
cols = range(1, 3)
# 传统
for row in rows:
  for col in cols:
    print(row, col)
# 推导式
cells = [(row, col) for row in rows for col in cols]
for row, col in cells:
  print(row, col)

字典推导式:

word = 'letters'
letter_counts = {letter: word.count(letter) for letter in word}
# {'e': 2, 'l': 1, 'r': 1, 't': 2, 's': 1}

集合推导式:

a_set = {number for number in range(1, 6) if number % 2 == 1}
# {1, 3, 5}

生成器推导式: (元组没有推导式)

number_thing = (number for number in range(1, 6))
type(number_thing)  # <class 'generator'>

for number in number_thing:
  print(number)

# 转换为list(一个生成器只运行一次,只在运行中产生值)
list(number_thing)  # [1, 2, 3, 4, 5]
list(number_thing)  # []

7. 函数

使用is判断None

def is_none(thing):
  if thing is None:
    print('It\'s None')
  elif thing:
    print('It\'s True')
  else:
    print('It\'s False')

is_none(None) # It's None
is_none([]) # It's False
is_none(1)  # It's True

参数
默认参数在函数被定义时就已经计算出来了

# bug
def buggy(arg, result=[]):
  result.append(arg)
  print(result)

buggy('a')  # ['a']
buggy('b')  # ['a', 'b']

# fix
def buggy(arg, result=None):
  if result is None:
    result = []
  result.append(arg)
  print(result)

buggy('a')  # ['a']
buggy('b')  # ['b']
def ours(first, middle, last):
  return {'first': first, 'middle': middle, 'last': last}

ours(1, 3, 7)  # 位置参数
ours(1, 3, last=5) # 关键字参数(关键字参数必须跟随在位置参数的后面)

# 使用*收集位置参数: 将一组可变数量的位置参数集合成参数值的元组
def print_args(required1, required2, *args):
  print(required1, required2, args)

print_args(1, 2)  # 1 2 ()
print_args(1, 2, 3, 4, 5) # 1 2 (3, 4, 5)

# 使用**收集关键字参数: 将关键字参数收集到一个字典中
def print_kwargs(required1, *args, **kwargs):
  print(required1, args, kwargs)

print_kwargs(1, 2, 3, a=4, b=5) # 1 (2, 3) {'b': 5, 'a': 4}

闭包:

def outer(num):
  def inner():
    return 'This is %d' % num
  return inner

a = outer(1)
b = outer(2)

print(a())  # This is 1
print(b())  # This is 2

lambda()函数:

def edit_story(words, func):
  for word in words:
    print(func(word))

edit_story(['hello', 'bye', 'see'], lambda word: word.capitalize() + '!')
# Hello!
# Bye!
# See!

8. 生成器

调用生成器函数,返回一个generator对象,供迭代器产生数据

def my_range(first=0, last=10, step=1):
  number = first
  while number < last:
    yield number
    number += step

for x in my_range(1, 5):
  print(x)

9. 装饰器

在不改变源代码的情况下修改已经存在的函数

def document_it(func):
  def new_func(*args, **kwargs):
    print('Running function:', func.__name__)
    print('arguments:', args, kwargs)
    result = func(*args, **kwargs)
    print('Result:', result)
    return result
  return new_func

@document_it
def add_ints(a, b):
  print('...execute...')
  return a + b

# 等价于
# def add_ints(a, b):
#   print('...execute...')
#   return a + b

# cooler_add_ints = document_it(add_ints)

add_ints(2, 4)

# Running function: add_ints
# arguments: (2, 4) {}
# ...execute...
# Result: 6

多个装饰器时,靠近def的装饰器最先执行

10. 命名空间和作用域

name = 'a'  # global

def print_global_name():
  print(name, id(name))

def print_local_name():
  name = 'b'  # local
  print(name, id(name))

def change_global_name():
  global name # 修改全局变量之前先在变量前面显式声明global
  name = 'b'
  print(name, id(name))

print_global_name() # a 61125472
print_local_name()  # b 61127936
print(name, id(name)) # a 61125472

change_global_name()  # b 61127936
print(name, id(name)) # b 61127936

11. try...except处理异常

short_list = [1, 2, 3]
while True:
  value = input('Position [q to quit]?')

  if value == 'q':
    break
  
  try:
    position = int(value)
    print(short_list[position])
  except IndexError as err:
    print('Bad index:', position)
  except Exception as other:
    print('Something else broken:', other)
上一篇 下一篇

猜你喜欢

热点阅读