(三)python语法之基础语法

目录

• 1.条件语句
• 2.循环语句
• 3.函数
• 4.异常
• 5.测试
• 6.文件处理
  • 读取文件
  • 写入文件
  • os
  • os.path
• 7.迭代器
  • 迭代器实现
  • itertools
• 8.生成器
• 9.闭包
• 10.装饰器
• 11.上下文管理器

1.条件语句

#1 if elif else
score = 55
if score > 80:
    print("优秀")
elif score > 60:
    print("及格")
else:
    print("未及格")

#2 条件表达式
result = "及格" if score>60 else "未及格"
print(result) # 未及格

2.循环语句

# for循环
for i in range(5):
    print(i, end=' ') # 0 1 2 3 4 
print('
')
  
# while循环
i = 0
while i < 5:
    print(i, end=' ') # 0 1 2 3 4
    i += 1
print('
')
    
# break语句
for i in range(5):
    if i > 2:
        break
    print(i, end=' ') # 0 1 2
print('
')

# continue语句
for i in range(5):
    if i % 2 == 0:
        continue
    print(i, end=' ') # 1 3 
print('
')

# for/else语句
for i in range(5):
    if i > 10:
        break
    print(i, end=' ') # 0 1 2 3 4
else:
    # else块在循环正常结束时执行
    print("我执行了")

3.函数

#1 函数定义与调用
def func(name, age=20, *args, **kw):
    print(name, age)
    print(args)
    print(kw)
func('Mary', 21, 1,2, a='A',b='B')
# Mary 21
# (1, 2)
# {'a': 'A', 'b': 'B'}

#2 lambda匿名函数
add = lambda x, y: x + y   
print(add(2,3)) # 5

# 在定义时绑定值
y = 10
add = lambda x, y=y: x + y 
print(add(2))   # 12

funcs = [lambda x: x+n for n in range(5)]
for func in funcs:
    print(func(5)) # 9 9 9 9 9

# 在定义时绑定值    
funcs = [lambda x, n=n: x+n for n in range(5)]
for func in funcs:
    print(func(5)) # 5 6 7 8 9

#3 偏函数
from functools import partial

int2 = partial(int, base=2)
print(int2('110'))       # 6

max10 = partial(max, 10) # 把10加入比较
print(max10(1,2,3))      # 10

#4 命令行参数
import sys
print(sys.argv[0])
print(sys.argv[1])

4.异常

#1 捕获异常
try:
    print(int('100years'))
except ValueError as e:
    print('ValueError:', e) 
else:
    # try语句没有异常时执行
    print('No error!')
finally:
    # 此处代码一定会执行
    print('Finally...')

#2 捕获多个异常
try:
    file = open('file.txt', 'rb')
except EOFError as e:
    print("An EOF error occurred.")
    raise e
except IOError as e:
    print("An error occurred.")
    raise e

#3 捕获所有异常
try:
    file = open('file.txt', 'rb')
except Exception as e:
    raise e

5.测试

# test.py
import unittest  

class TestList(unittest.TestCase):
    # 测试方法以test开头
    def test_pop(self):
        l = [1, 2, 3]
        r = l.pop()
        self.assertEqual(l, [1,2])
        self.assertTrue(r == 3)
        self.assertFalse(r == 1)
        
    def test_index(self):
        l = [1, 2, 3]
        n = l.index(2)
        self.assertEqual(n, 1)
        with self.assertRaises(ValueError):
            n = l.index(4)   
    
if __name__ == '__main__':
    unittest.main()
    
# python test.py -v

6.文件处理

读取文件

#1 读取文本文件
f = open('a.txt', 'r', encoding='gbk') 
data = f.read()
print(data)
f.close()

with open('test.txt', 'r') as f:
    # 读取文件全部内容,返回str
    print(f.read())       
    
with open('test.txt', 'r') as f:    
    # 每次最多读取6个字节的内容,返回str
    print(f.read(6))      
    
with open('test.txt', 'r') as f:    
    # 每次读取一行内容,返回str
    print(f.readline())   
    
with open('test.txt', 'r') as f:   
    # 读取文件全部内容,按行返回list
    print(f.readlines())  

#2 读取二进制文件
with open('test.bin', 'rb') as f:
    data = f.read(16)
    text = data.decode('utf-8')

写入文件

#1 写入文本文件
with open('test1.txt', 'w') as f:
    f.write('Hello, world!')

#2 将print输出重定向到文件    
with open('test2.txt', 'wt') as f:
    print('Hello World!', file=f)

#3 写入二进制文件
with open('text3.bin', 'wb') as f:
    text = 'Hello World'
    f.write(text.encode('utf-8'))

os

import os 

os.name           # 操作系统类型
os.environ        # 操作系统中的环境变量

os.getcwd()       # 当前工作目录
os.listdir('.')   # 列举目录的文件
os.stat('a.txt')  # 获取文件元数据

os.rename('a.txt', 'a.py') # 重命名 
os.remove('a.txt')         # 删除文件

os.mkdir('test')         # 创建目录 
os.chdir('test')         # 切换目录
os.makedirs('dir1/dir2') # 创建多级目录

# 递归遍历文件夹
for root, dirs, files in os.walk('.'):
    for name in files:
        print(os.path.join(root, name))
    for name in dirs:
        print(os.path.join(root, name))

os.path

import os

os.path.abspath('.')  # 查看绝对路径
os.path.exists('.')   # 文件或目录是否存在
os.path.isdir('.')    # 是否是目录
os.path.isfile('.')   # 是否是文件
os.path.islink('.')   # 是否是链接
os.path.realpath('.') # 返回被链接的原始文件
os.path.getsize('a.txt')  # 获取文件大小
os.path.getmtime('a.txt') # 获取修改日期

# 目录合并
print(os.path.join('test', 'test.txt'))      
# test/test.txt 

# 目录分离
path = '/home/test/test.txt'
print(os.path.basename(path)) #'test.txt' 
print(os.path.dirname(path))  #'/home/test'

print(os.path.split(path))    
#('/home/test', 'test.txt') 
print(os.path.splitext(path)) 
#('/home/test/test', '.txt')

7.迭代器

迭代器实现

class Fib(object):  
    def __init__(self, num):
        self.a, self.b = 0, 1
        self.num = num
        
    def __iter__(self):       
        return self           
    
    def __next__(self):
        self.a, self.b = self.b, self.a + self.b 
        if self.a > self.num: 
            raise StopIteration()
        return self.a  

fib = Fib(10)
for i in fib:
    print(i) # 1 1 2 3 5 8

itertools

import itertools

#1 count(1)从1开始创建一个无限的迭代器
num = itertools.count(1)
for i in num:
    if i > 5:
        break
    print(i, end=' ') # 1 2 3 4 5 

#2 cycle()把传入的序列无限重复下去
alpha = itertools.cycle('ABC') 
n = 0
for i in alpha:
    if n > 6:
        break
    n += 1
    print(i, end=' ') # A B C A B C A

#3 repeat()把一个元素无限重复下去，第二个参数可以限定重复次数
alpha = itertools.repeat('A', 3)
for i in alpha:
    print(i, end=' ') # A A A 

#4 takewhile()根据条件截取出一个有限的序列
num = itertools.count(1)
part = itertools.takewhile(lambda x: x < 5, num)
print(list(part)) # [1, 2, 3, 4]

#5 chain()把一组迭代对象连接起来
for i in itertools.chain('ABC', 'DEF'):
    print(i, end=' ') # A B C D E F 

#6 groupby()把迭代器中相邻的重复元素挑出来放在一起
for key, group in itertools.groupby('ABBCCC'):
    print(key, list(group))
# A ['A']
# B ['B', 'B']
# C ['C', 'C', 'C']

#7 groupby()的挑选规则通过函数完成,只要作用于函数的两个元素的返回值相等，就被认为是一组，而函数返回值作为组的key
for key, group in itertools.groupby('AaBBbcCcC', lambda c: c.upper()):
    print(key, list(group))
# A ['A', 'a']
# B ['B', 'B', 'b']
# C ['c', 'C', 'c', 'C']

8.生成器

#1 使用推导式构建生成器
g = (x * x for x in range(5))

for i in g:
    print(i) # 0 1 4 9 16

#2 使用yield关键字构建生成器
def g():
    for i in range(5):
        yield i * i

for i in g():
    print(i) # 0 1 4 9 16

9.闭包

#1 闭包
def lazy_sum(*args):
    def sum():
        _sum = 0
        for n in args:
            _sum += n
        return _sum
    return sum

f = lazy_sum(1,2,3,4,5)
print(f()) # 15

#2 循环变量的当前值未被捕获：
def count():
    fs = []
    for i in range(1, 4):
        def f():
            return i*i
        fs.append(f) 
    return fs

f1, f2, f3 = count() 
print(f1(), f2(), f3()) # 9 9 9

#3 循环变量的当前值被捕获：
def count():
    fs = []
    for i in range(1, 4):
        def f(_i=i):       
            return _i*_i
        fs.append(f)    
    return fs

f1, f2, f3 = count()    
print(f1(), f2(), f3()) # 1 4 9

#4 利用闭包实现计数器，每次调用返回递增整数：
def createCounter():
    fs = [0]
    def counter():
        fs[0] = fs[0] + 1
        return fs[0]
    return counter

f = createCounter()
for i in range(5):
    print(f()) # 1 2 3 4 5

10.装饰器

import functools
import time

#1 装饰器 
def log(func):
    @functools.wraps(func)
    def wrapper(*args, **kw):
        print(func.__name__) 
        return func(*args, **kw)
    return wrapper

@log
def func():
    print('Hello')
    
func()
# func
# Hello

#2 传入参数的装饰器
def log(text):
    def decorator(func):
        @functools.wraps(func)
        def wrapper(*args, **kw):
            print(text, func.__name__)
            return func(*args, **kw)
        return wrapper
    return decorator

@log('execute')
def func():
    print('Hello')

func()
# execute func
# Hello

#3 打印函数的执行时间
def metric(fn):
    @functools.wraps(fn)
    def wrapper(*args, **kw):
        start = time.time()
        f = fn(*args, **kw)  
        print(time.time() - start)
        return f
    return wrapper

@metric
def func():
    sum = 0
    for i in range(1000000):
        sum += i*i*i*i
        sum -= i*i*i*i
    print('sum: ', sum)
    
func()
# sum:  0
# 0.25935

#4 装饰器类
class logit(object): 
    def __init__(self, msg):
        self.msg = msg
        print('init:', self.msg)
    
    def __call__(self, func):
        def wrapper(*args, **kw):       
            print(func.__name__) 
            print(self.msg)
            return func(*args, **kw)
        return wrapper
    
@logit('log')
def func():
    print("Hello")

func()
# init: log
# func
# log
# Hello

11.上下文管理器

#1 基于类，定义enter和exit方法
class File(object):
    def __init__(self, name, method):
        self.file = open(name, method)
    
    def __enter__(self):
        return self.file
    
    #exit返回True清空处理异常，否则抛出异常
    def __exit__(self, type, val, trace): 
        self.file.close()
        return True

with File('test.py', 'r') as f:
    f.undefined_function()
    print('Hello')  # 这里不执行了

#2 基于生成器
from contextlib import contextmanager

@contextmanager
def open_file(name):
    f = open(name, 'w')
    yield f
    f.close()
    
with open_file('test.txt') as f:
    f.write('hello!')
    
#3 首尾代码固定时：
@contextmanager
def tag(name):
    print("<%s>" % name)
    yield
    print("</%s>" % name)
    
with tag("h1"):
    print("hello world")
# <h1>
# hello world
# </h1>