python 有用的Python代码片段

Posted 2021-05-09

# 获取文件扩展名(e.g. .exe .txt)
extension = os.path.splitex(filename)[1]

# lazy property
# 惰性属性装饰器，第一次使用求值，之后使用直接从object.__dict__中取属性
class lazy_property(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, cls):
        val = self.func(instance)
        setattr(instance, self.func.__name__, val)
        return val

# key，value互换
>>> m = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
>>> {v: k for k, v in m.items()} 
{1: 'a', 2: 'b', 3: 'c', 4: 'd'}

# list 分组
>>> a=[1,2,3,4,5,6,7,8,9,0]
>>> [a[i:i+3] for i in range(0,len(a),3)]
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [0]]

# 矩阵转置
>>> a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> print([[row[col] for row in a] for col in range(len(a[0]))])
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
>>> list(zip(*a))[:]  # zip(*a) 即可 不过返回的是迭代器，而且内部是元祖
[(1, 4, 7), (2, 5, 8), (3, 6, 9)]
>>> list(map(list，zip(*a)))[:]  # 通过map将tuple转换为list，map返回的也是迭代器
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]

# namedtuple 简单的结构体
>>> from collections import namedtuple
>>> Point = namedtuple('Point', ['x', 'y'])                                                        
>>> Point.__doc__                   # docstring for the new class                                  
'Point(x, y)'                                                                                      
>>> p = Point(11, y=22)             # instantiate with positional args or keywords                 
>>> p[0] + p[1]                     # indexable like a plain tuple                                 
33                                                                                                 
>>> x, y = p                        # unpack like a regular tuple                                  
>>> x, y                                                                                           
(11, 22)                                                                                           
>>> p.x + p.y                       # fields also accessable by name                               
33                                                                                                 
>>> d = p._asdict()                 # convert to a dictionary                                      
>>> d['x']                                                                                         
11                                                                                                 
>>> Point(**d)                      # convert from a dictionary                                    
Point(x=11, y=22)                                                                                  
>>> p._replace(x=100)               # _replace() is like str.replace() but targets named fields    
Point(x=100, y=22)  

# defaultdict 
# 假设我们的字典中的value是一个list。我们要先判断key是否已经存在，如果不存在，则新建一个list并赋值给key，
# 如果已经存在，则调用list的append方法，将值添加进去。

d = {}
for key, value in pairs:
    if key not in d:
        d[key] = []
    d[key].append(value)
    
d = defaultdict(list) # 这个方法更好
    for key, value in pairs:
        d[key].append(value)

      
# 字典的get方法
# 如果元素在字典中没有找到的话，则设为指定值
grade = record['Tom']
if grade is None:
    grade =0
    
grade=record.get('Tom',0) 

# 列表每个元素乘以一个值
l = [1, 2, 3, 4, 5]
l = [x * 2 for x in l] 

# 复制列表 
> a=[1,2,3] 
> b=list(a) 
> b=a[:]

# a or b
a = b if a is None else a
a= a or b  # or 会在求得一个真值的时候返回

# 类型检查装饰器
def accepts(*types):
    def check_accepts(f):
        assert len(types) == f.func_code.co_argcount
        def new_f(*args, **kwds):
            for (a, t) in zip(args, types):
                assert isinstance(a, t), \
                       "arg %r does not match %s" % (a,t)
            return f(*args, **kwds)
        new_f.func_name = f.func_name
        return new_f
    return check_accepts
Usage:

@accepts(int, (int,float))
def func(arg1, arg2):
    return arg1 * arg2

func(3, 2) # -> 6
func('3', 2) # -> AssertionError: arg '3' does not match <type 'int'>