Python 黑魔法（持续收录）
zip 对矩阵进行转置
a = [[1, 2, 3], [4, 5, 6]]
print(list(map(list, zip(*a))))
zip 反转字典
a = dict(a=1, b=2, c=3)
print(dict(zip(a.values(), a.keys())))
将list分成n份
print(list(zip(*(iter([1, 2, 3, 4, 5, 6]),) * 3)))
# [(1, 2, 3), (4, 5, 6)]
all & any 函数
- all:如果iterable的所有元素不为0、''、False或者iterable为空，all(iterable)返回True，否则返回False
- any: 如果所有元素中有一个值不是0、''或False，那么结果就为True，否则为False
print(any([]))
# False
print(all([]))
# True
print(all([1,2,3,0]))
# False
Concatenate long strings elegantly across line breaks in code
my_long_text = ("We are no longer the knights who say Ni! "
"We are now the knights who say ekki-ekki-"
"ekki-p'tang-zoom-boing-z'nourrwringmm!")
print(my_long_text)
# We are no longer the knights who say Ni! We are now the knights who say ekki-ekki-ekki-p'tang-zoom-boing-z'nourrwringmm!
calling different functions with same arguments based on condition
def product(a, b):
return a * b
def subtract(a, b):
return a - b
b = True
print((product if b else subtract)(1, 1))
Sort dict keys by value
d = {'apple': 10, 'orange': 20, 'banana': 5, 'rotten tomato': 1}
print(sorted(d, key=d.get))
# ['rotten tomato', 'banana', 'apple', 'orange']
exec
exec("print('Hello ' + s)", {'s': 'World!'})
# exec can be used to execute Python code during runtime variables can be handed over as a dict
unpacking
[(c, *d, [*e]), f, *g] = [[1, 2, 3, 4, [5, 5, 5]], 6, 7, 8]
print(c, d, e, f, g)
# 1 [2, 3, 4] [5, 5, 5] 6 [7, 8]
flatten list
import itertools
a = [[1, 2], [3, 4], [[5,6],[7,8]]]
print(list(itertools.chain(*a)))
# [1, 2, 3, 4, [5, 6], [7, 8]]
把嵌套的也flatten？
a = [[1, 2], [3, 4], [[5, 6], [7, 8]]]
a = eval('[%s]' % repr(a).replace('[', '').replace(']', ''))
print(a)
# [1, 2, 3, 4, 5, 6, 7, 8]
更简单？
a = [[1, 'a', ['cat'], 2], [[[3], 'a', 'm', [1, 2, 3], [1, [1, 2, 3]]]], 'dog']
flatten = lambda L: eval(str(L).replace('[', '*[')[1:])
flatten(a)
dict求交
dctA = {'a': 1, 'b': 2, 'c': 3}
dctB = {'b': 4, 'c': 3, 'd': 6}
# loop over dicts that share (some) keys in Python3
for ky in dctA.keys() & dctB.keys():
print(ky)
# loop over dicts that share (some) keys and values in Python3
for item in dctA.items() & dctB.items():
print(item)
split a string max times
"""split a string max times"""
string = "a_b_c"
print(string.split("_", 1))
# ['a', 'b_c']
"""use maxsplit with arbitrary whitespace"""
s = "foo bar foobar foo"
print(s.split(None, 2))
# ['foo', 'bar', 'foobar foo']
字典合并
d1 = {'a': 1}
d2 = {'b': 2}
# python 3.5
print({**d1, **d2})
print(dict(d1.items() | d2.items()))
d1.update(d2)
print(d1)
Find Index of Min/Max Element
lst = [40, 10, 20, 30]
def minIndex(lst):
return min(range(len(lst)), key=lst.__getitem__) # use xrange if < 2.7
def maxIndex(lst):
return max(range(len(lst)), key=lst.__getitem__) # use xrange if < 2.7
print(minIndex(lst))
print(maxIndex(lst))
remove duplicate items from list and keep order
from collections import OrderedDict
items = ["foo", "bar", "bar", "foo"]
print(list(OrderedDict.fromkeys(items).keys()))
set global variables from dict
def foo():
d = {'a': 1, 'b': 'var2', 'c': [1, 2, 3]}
globals().update(d)
foo()
print(a, b, c)
Sort a list and store previous indices of values
l = [4, 2, 3, 5, 1]
print("original list: ", l)
values, indices = zip(*sorted((a, b) for (b, a) in enumerate(l)))
# now values contains the sorted list and indices contains
# the indices of the corresponding value in the original list
print("sorted list: ", values)
print("original indices: ", indices)
# note that this returns tuples, but if necessary they can
# be converted to lists using list()
None
from collections import defaultdict
tree = lambda: defaultdict(tree)
users = tree()
users['harold']['username'] = 'chopper'
users['matt']['password'] = 'hunter2'
for_else 跳出多层循环
for i in range(5):
for j in range(6):
print(i * j)
if i * j == 20:
break
else:
continue
break