Python学习之17个关于Python的小技巧

"""pythonic way of value swapping"""
a, b=5,10
print(a,b)
a,b=b,a
print(a,b)

a=["python","is","awesome"]
print(" ".join(a))

"""most frequent element in a list"""
a=[1,2,3,1,2,3,2,2,4,5,1]
print(max(set(a),key=a.count))
"""using Counter from collections"""
from collections import Counter
cnt=Counter(a)
print(cnt.most_commin(3))

from collections import Counter
Counter(str1)==Counter(str2)

"""reversing string with special case of slice step param"""
 a ='abcdefghij k lmnopqrs tuvwxyz 'print(a[ ::-1] )
 """iterating over string contents in reverse efficiently."""
 for char in reversed(a):
 print(char )
 """reversing an integer through type conversion and slicing ."""
 num = 123456789
 print( int( str(num)[::1]))

 """reversing list with special case of slice step param"""
 a=[5,4,3,2,1]
 print(a[::1])
 """iterating over list contents in reverse efficiently ."""
 for ele in reversed(a):
 print(ele )

"""transpose 2d array [[a,b], [c,d], [e,f]] -> [[a,c,e], [b,d,f]]"""
original = [['a', 'b'], ['c', 'd'], ['e', 'f']]
transposed = zip( *original )
print(list( transposed) )

""" chained comparison with all kind of operators"""
b =6
print(4< b < 7 )
print(1 == b < 20)

"""calling different functions with same arguments based on condition"""
def product(a, b): 
 return a * b
def add(a, b): 
 return a+ b
b =True
print((product if b else add)(5, 7))

 """a fast way to make a shallow copy of a list"""
 b=a
 b[0]= 10
 """ bothaandbwillbe[10,2,3,4,5]"""
 b = a[:]b[O] = 10
 """only b will change to [10, 2, 3, 4, 5] """
 """copy list by typecasting method"""
 a=[l,2,3,4,5]
print(list(a))
 """using the list.copy( ) method ( python3 only )""" 
 a=[1,2,3,4,5]
 print(a.copy( ))
 """copy nested lists using copy. deepcopy"""
 from copy import deepcopy
 l=[l,2],[3,4]]
 l2 = deepcopy(l)
print(l2)

""" returning None or default value, when key is not in dict""" 
d = ['a': 1, 'b': 2]
print(d.get('c', 3))

"""Sort a dictionary by its values with the built-in sorted( ) function and a ' key' argument ."""
 d = {'apple': 10, 'orange': 20, ' banana': 5, 'rotten tomato': 1)
 print( sorted(d. items( ), key=lambda x: x[1]))
 """Sort using operator . itemgetter as the sort key instead of a lambda"""
 from operator import itemgetter
 print( sorted(d. items(), key=itemgetter(1)))
 """Sort dict keys by value"""
 print( sorted(d, key=d.get))

"""else gets called when for loop does not reach break statement"""
a=[1,2,3,4,5]
for el in a: 
 if el==0: 
 break
else: 
 print( 'did not break out of for loop' )

 """converts list to comma separated string"""
items = [foo', 'bar', 'xyz']
print (','.join( items))
"""list of numbers to comma separated"""
numbers = [2, 3, 5, 10]
print (','.join(map(str, numbers)))
"""list of mix data"""
data = [2, 'hello', 3, 3,4]
print (','.join(map(str, data)))

"""merge dict's"""
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__ )
def maxIndex(lst):
 return max( range( len(lst)), key=lst.. getitem__ )
print( minIndex(lst)) 
print( maxIndex(lst))

 """remove duplicate items from list. note: does, not preserve the original list order"""
items=[2,2,3,3,1]
newitems2 = list(set( items)) 
print (newitems2)
"""remove dups and, keep. order"""
from collections import OrderedDict
items = ["foo", "bar", "bar", "foo"]
print( list( orderedDict.f romkeys(items ).keys( )))