基于编程人员Python学习第一章节

>>> print(r'\\\t\\')
\\\t\\

>>> print('''line1
... line2
... line3''')
line1
line2
line3

>>> 'ABC'.encode('ascii')
b'ABC'
>>> '中文'.encode('utf-8')
b'\xe4\xb8\xad\xe6\x96\x87'

>>> b'ABC'.decode('ascii')
'ABC'
>>> b'\xe4\xb8\xad\xe6\x96\x87'.decode('utf-8')
'中文'
>>> b'\xe4\xb8\xad\xff'.decode('utf-8', errors='ignore')
'中'

>>> print('%2d-%02d' % (3, 1))
 3-01
>>> print('%.2f' % 3.1415926)
3.14、

classmates = ['Michael', 'Bob', 'Tracy']　　　　集合list  append 添加，pop 删除

classmates = ('Michael', 'Bob', 'Tracy')

>>> classmates[-3]
'Michael'

#只有一个元素tuple 定义加上逗号 ，
>>> t = (1,)
>>> t
(1,)

#“可变的”tuple 内部list改变，实际指向list位置未变
>>> t = ('a', 'b', ['A', 'B'])
>>> t[2][0] = 'X'
>>> t[2][1] = 'Y'
>>> t
('a', 'b', ['X', 'Y'])

age = 20
if age >= 6:
    print('teenager')
elif age >= 18:
    print('adult')
else:
    print('kid')

>>> d = {'Michael': 95, 'Bob': 75, 'Tracy': 85}
>>> d['Michael']
95

#设定默认值
>>> d.get('Thomas', -1)
-1

#删除
>>> d.pop('Bob')
75

#重复元素在set中自动被过滤：  add(key)　　remove(key)
>>> s = set([1, 2, 3])
>>> s
{1, 2, 3}

def my_abs(x):
    if x >= 0:
        return x
    else:
        return -x
#返回多个值 return a,b,c

#默认参数必须指向不变对象！
def add_end(L=None):
    if L is None:
        L = []
    L.append('END')
    return L

# *args 可变参数 **关键字参数
def f1(a, b, c=0, *args, **kw):
    print('a =', a, 'b =', b, 'c =', c, 'args =', args, 'kw =', kw)

>>> f1(1, 2, 3, 'a', 'b', x=99)
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {'x': 99}

# *,d 命名关键字参数
def f2(a, b, c=0, *, d, **kw):
    print('a =', a, 'b =', b, 'c =', c, 'd =', d, 'kw =', kw)

>>> f2(1, 2, d=99, ext=None)
a = 1 b = 2 c = 0 d = 99 kw = {'ext': None}

#对于任意函数，都可以通过类似func(*args, **kw)的形式调用它，无论它的参数是如何定义的

>>> L = ['Michael', 'Sarah', 'Tracy', 'Bob', 'Jack']
>>> L[1:3]　　# L[1] L[2]
['Sarah', 'Tracy']

>>> L = list(range(100))

>>> L[-10:]　　# L[-10:100] 后10个
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

>>> L[:10:2]　　# L[0:10:2] 前10个数，每两个取一个
[0, 2, 4, 6, 8]

>>> L[:]　　# L[0:100:1] copy 复制
[0, 1, 2, 3, ..., 99]

　>>> L[::-1]　　#相当于 L[-1:-101:-1]  if s<0 then L[-1:-len(L)-1:-1]
　[99, 98, 97, ..., 0]

>>> from collections import Iterable
>>> isinstance('abc', Iterable) # str是否可迭代
True
>>> isinstance([1,2,3], Iterable) # list是否可迭代
True
>>> isinstance(123, Iterable) # 整数是否可迭代
False

>>> for i, value in enumerate(['A', 'B', 'C']):
...     print(i, value)
...
0 A
1 B
2 C

>>> [x * x for x in range(1, 11) if x % 2 == 0]
[4, 16, 36, 64, 100]

>>> L = ['Hello', 'World', 'IBM', 'Apple']
>>> [s.lower() for s in L]
['hello', 'world', 'ibm', 'apple']

>>> L = [x * x for x in range(10)]
>>> L
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> g = (x * x for x in range(10))
>>> g
<generator object <genexpr> at 0x1022ef630>
# 使用next(g) or for n in g 可迭代对象g

#斐波拉契数
def fib(max):
    n, a, b = 0, 0, 1
    while n < max:
        yield b
        a, b = b, a + b
        n = n + 1
    return 'done'

>>> from collections import Iterator
>>> isinstance((x for x in range(10)), Iterator)
True
>>> isinstance([], Iterator)
False
>>> isinstance({}, Iterator)
False
>>> isinstance('abc', Iterator)
False

#Iterable。 for 循环迭代
#Iterator。 next 迭代器
# Iterable 转换 Iterator
>>> isinstance(iter([]), Iterator)
True
>>> isinstance(iter('abc'), Iterator)
True

#map 传入函数依次作用到序列每个元素，返回Iterator结果集
>>> def f(x):
...     return x * x
...
>>> r = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> list(r)
[1, 4, 9, 16, 25, 36, 49, 64, 81]
# r 为Iterator惰性序列，list() 是整个序列返回

#reduce  reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)
>>> from functools import reduce
>>> def add(x, y):
...     return x + y
...
>>> reduce(add, [1, 3, 5, 7, 9])
25

#map reduce 结合使用
from functools import reduce
DIGITS = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
def char2num(s):
    return DIGITS[s]
def str2int(s):
    return reduce(lambda x, y: x * 10 + y, map(char2num, s))

#filter filter()把传入的函数依次作用于每个元素，然后根据返回值是True还是False决定保留还是丢弃
def not_empty(s):
    return s and s.strip()

list(filter(not_empty, ['A', '', 'B', None, 'C', '  ']))
# 结果: ['A', 'B', 'C']

#sorted sorted()函数对list进行排序
>>> sorted(['bob', 'about', 'Zoo', 'Credit'], key=str.lower, reverse=True)
['Zoo', 'Credit', 'bob', 'about']

def lazy_sum(*args):
    def sum():
        ax = 0
        for n in args:
            ax = ax + n
        return ax
    return sum
>>> f = lazy_sum(1, 3, 5, 7, 9)
>>> f
<function lazy_sum.<locals>.sum at 0x101c6ed90>
>>> f()
25

#典型闭包错误
def count():
    fs = []
    for i in range(1, 4):
        def f():
             return i*i
        fs.append(f)
    return fs

f1, f2, f3 = count()

>>> f1()
9
>>> f2()
9
>>　f3()
9

##返回闭包时牢记一点：返回函数不要引用任何循环变量，或者后续会发生变化的变量

修改后：

def count():
    def f(j):
        def g():
            return j*j
        return g
    fs = []
    for i in range(1, 4):
        fs.append(f(i)) # f(i)立刻被执行，因此i的当前值被传入f()
    return fs

>>> list(map(lambda x: x * x, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
[1, 4, 9, 16, 25, 36, 49, 64, 81]
#lambda 不包含return，返回即使return结果

import functools

def log(text):
    def decorator(func):
        @functools.wraps(func)
        def wrapper(*args, **kw):
            print('%s %s():' % (text, func.__name__))
            return func(*args, **kw)
        return wrapper
    return decorator

@log('execute')
def now():
    print('2018-8-13')

>>> now()
execute now():
2018-8-13

>>> import functools
>>> int2 = functools.partial(int, base=2)
>>> int2('1000000')
64
#相当于下面这样调用
kw = { 'base': 2 }
int('10010', **kw)