某个不愿理透露姓名的大大跟我说:
小伙子啊,你的代码有潜在的 bug 啊,i += x
会粗问题的。我思考了良久,都不得要领。
那么请问,什么情况下 "i += x" 不等于 "i = i + x"?
大家讲道理2017-04-17 13:50:00
Look at the comparison of the two pieces of code below:
Code 1:
>>> l1=range(3)
>>> l2=l1
>>> l2+=[4]
>>> l1
[0, 1, 2, 4]
>>> l2
[0, 1, 2, 4]
Code 2:
>>> l2=l1
>>> l2=l2+[4]
>>> l1
[0, 1, 2]
>>> l2
[0, 1, 2, 4]
See here for a detailed explanation: http://stackoverflow.com/questions/2347265/why-does-behave-unexpectedly-on-lists
伊谢尔伦2017-04-17 13:50:00
Obviously, just like what the 2nd floor said.
Code 1 l2 = l1, l2 += [4], all operations are l1, similar to pointers, referencing Shenma, if you don’t understand it, just think about it.
Code 2 l2 = l1, l2 = l2 + [4], this is obviously a reassignment of l2. You can write l3 = l2 + [4], l2 += [4]. Then you will know the result.
You can learn about python’s deep copy and shallow copy
There is a very good answer in stackoverflow on the second floor. Just use id() to check the memory address
>>> l = []
>>> id(l)
13043192
>>> l += [3]
>>> id(l)
13043192
>>> l = l + [3]
>>> id(l)
13059216
Don’t worry too much, just look at the memory address and it will be clear at a glance.
id is a built-in function of python, what's id? ...
《'id' is a bad variable name in Python》
id() is a fundamental built-in:
Help on built-in function id in module builtin:
id(...)
id(object) -> integer
Return the identity of an object. This is guaranteed to be unique among
simultaneously existing objects. (Hint: it's the object's memory
address.)
ringa_lee2017-04-17 13:50:00
What I can think of is that some classes implement __iadd__
and behave differently from __add__
. List's +=
is an example. Ordinary +
will return a new list instance, while +=
is Directly manipulate the list itself.
If you can confirm that i
and x
are two numbers, saying that i += x
will cause problems is a bit nitpicking and dogmatism.
伊谢尔伦2017-04-17 13:50:00
Mainly look at i, if i is an expression
In i+=x, i is only calculated once
i = i+x i needs to be calculated 2 times
Then, if there are modification variables in the i expression, the values of the two i in i = i+x may be different
a[i++] += 5 // i 执行一次求值
a[i++] = a[i++] + 5 // i 执行两次求值
This example should be common in various grammar books
天蓬老师2017-04-17 13:50:00
The above is for reference types, there should be no difference for value types.
PHP中文网2017-04-17 13:50:00
The core issue is to prevent side effects. Make your code behave consistently.
迷茫2017-04-17 13:50:00
The poster here has some misunderstandings, i += x, this is a kind of syntax sugar, what does it really look like after expansion?
In C language, the true form of i += x is i = i+x. Here i is reassigned, so the pointer address of i will change.
But this is not the case in Python. The true form of i += x is i.extend(x), where the pointer address of i has not changed, only the value of i has been changed.
refer: http://stackoverflow.com/questions/2347265/why-does-behave-unexpectedly-on-lists
thank to: @WKPlus
高洛峰2017-04-17 13:50:00
What Floor 4 said is so right. There is a chance that the MD5 values of two different files are the same. The question is, have you ever encountered them?