问题
I accidentally wrote:
total_acc =+ accuracy
instead of:
total_acc += accuracy
I searched the net and could not find anything. So what happened, why does Python think I mean what I am typing?
Computers trust us too much. :)
回答1:
This is the same as if you were to do like total_acc = -accuracy, except positive instead of negative. It basically is the same as total_acc = accuracy though, as adding a + before a value does not change it.
This is called an unary operator as there is only one argument (ex: +a) instead of two (ex: a+b).
This link explains it a little more.
回答2:
If you are interested in catching this type of errors early, you can do that with static code analysis. For example, flake8:
$ cat test.py
total_acc = 0
accuracy = 10
total_acc =+ accuracy
$ flake8 test.py
test.py:4:12: E225 missing whitespace around operator
In this case, it is complaining about the extra space after the +, thinking that you actually meant total_acc = +accuracy. This would have helped you to discover the problem earlier.
FYI, pylint would catch that too.
回答3:
It thinks you're doing total_acc = +accuracy, which sets total_acc equal to accuracy. + before a variable without another value causes the variable's __pos__ method to be called. For most types, this is a nop, but there are certain types, e.g. Decimal that implement __pos__.
来源：https://stackoverflow.com/questions/35231789/python-operator-is-syntactically-correct