1 yuan group containing a single element
Some functions in Python argument types as tuples, within which there is an element, which is created is wrong:
c = (5) # NO!
！
It actually creates a cosmetic element 5, a must to add after the element 逗号:
c = (5,) # YES!
2 default parameter is set to null
Contains the default function arguments, if the type of container and set to null:
def f(a,b=[]): # NO!
print(b)
return b
ret = f(1)
ret.append(1)
ret.append(2)
# 当再调用f(1)时，预计打印为 []
f(1)
# 但是却为 [1,2]
This is the pit default parameters of the variable type, be sure to set the parameters of such default is None:
def f(a,b=None): # YES!
pass
3 shared variables unbound Pit
Sometimes you want more than one function to share a global variable, but it is trying to modify local variables within a function:
i = 1
def f():
i+=1 #NO!
def g():
print(i)
It should be displayed in the f function is declared ias global variables:
i = 1
def f():
global i # YES!
i+=1
4 copy of the list of fast pit
In python *with a list of actions to achieve rapid replication elements:
a = [1,3,5] * 3 # [1,3,5,1,3,5,1,3,5]
a[0] = 10 # [10, 2, 3, 1, 2, 3, 1, 2, 3]
If the list elements such as lists or dictionaries composite type:
a = [[1,3,5],[2,4]] * 3 # [[1, 3, 5], [2, 4], [1, 3, 5], [2, 4], [1, 3, 5], [2, 4]]
a[0][0] = 10 #
The results may surprise you, others a[1[0], also be modified to 10
[[10, 3, 5], [2, 4], [10, 3, 5], [2, 4], [10, 3, 5], [2, 4]]
* This is because the compound is a pale reference to the copied object, i.e. id (a [0]) and id (a [2]) are equal house number. If you want to achieve the effect of deep copy, do the following:
a = [[] for _ in range(3)]
5 list of deleted pit
Delete elements in a list, this element may be repeated several times in the list:
def del_item(lst,e):
return [lst.remove(i) for i in e if i==e] # NO!
Consider deleting this sequence elements [1,3,3,3,5] 3, wherein only two deleted found:
del_item([1,3,3,3,5],3) # 结果：[1,3,5]
The right approach:
def del_item(lst,e):
d = dict(zip(range(len(lst)),lst)) # YES! 构造字典
return [v for k,v in d.items() if v!=e]
These are the five common pit, hoping to see friends here can avoid these pits.
