Python's unboundlocalerror:local variable ' xxx ' referenced before Assignment__python

When we write Python, we sometimes run into situations where we've already defined the variable n outside the function, printed it out inside the function, and then made the variable go up, and then ran into this error:

unboundlocalerror:local variable ' xxx ' referenced before assignment

As shown in the following code slice:

N=0
def func ():
	print n
	n+=1

func ()

This error occurs when the result is run.

This is because after you have modified the variable assignment inside the function, the variable is considered a local variable instead of a global variable by the Python interpreter, and when the program executes to N+=1, because the statement is assigned to n, so n becomes a local variable, then when you execute print N, Because n is not defined as a local variable, this error is thrown naturally.

Consider the following code slice:

N=0
def func ():
	print n

func ()

Because the function does not have another value assigned to n, then print n prints the value of the global variable N.

So how do we get to the operation of printing and then assigning values inside a function? The conclusion is to use the Global keyword to declare that n is a global variable inside the function. The code is as follows:

N=0
def func ():
	global n
	print n
	n+=1

func ()
print n

The results are as follows:

0
1
[finished in 1.0s]

At this point, N becomes a global variable, and there is no problem in modifying the variable inside the function.