python 中的 scope 不一致

Question

I have tried creating a function like this:

n = ["one", "two"]
result=""
def join_strings(words):
  for word in words:
    result+=word
  return result
print (join_strings(n))

This does not work and outputs:

UnboundLocalError: local variable 'result' referenced before assignment

The result variable has to be inside the function for it to work.

However, when I have this simple function:

x=3
def price():
  return (x+3)
print (price())

This works and prints the price, even though x is outside of the function. Why is this happening?

Answer 1

Actually there is no inconsistency as the examples you gave are different from each other. It would still fail in the second function if you have tried to assign x to itself, like:

>>> x = 3
>>> def price():
...     x +=3
...     return x
...
>>> price()
UnboundLocalError: local variable 'x' referenced before assignment

Instead of assigning back to x if you choose another name, it would run with no problem:

>>> def price():
...     y  = x + 3
...     return y
...
>>> price()
6

But, why it happens?

It's because Python's scoping rules. You can read the value of a variable outside of function but you can't change it**. When you do x += 3 , which is same as x = x + 3 for integers, that means " I have a variable x that I have write access to it in the current scope. " You don't have such variable, thus as the error says: you are referencing a "local variable" before assignment.

Is there a way to modify them in the function?

Yes. You can use global keyword, changes will be applied to your global variable:

>>> x = 3
>>> def price():
...     global x
...     x += 3
...     return x
...
>>> x
3
>>> price()
6
>>> x
6

** By changing , I mean assigning something else to it so it's id will change.

Answer 2

The difference is that in the second example you aren't trying to modify or reassign x . (You'll get the same error if you say something like x += 3 in the price function.) Once you use an assignment operator, you're binding a new value to that name, shadowing the outer scope.

If you want to be able to modify something from an outer scope, put it inside a mutable container (like a list); you can then modify the contents without reassigning that variable.

result = [""]
def join_strings(words):
    for word in words:
        result[0] += word
    return result[0]

If you just want to be able to reference the result value, without modifying it, that's fine too, but you have to assign it to a new variable then:

result = ""
def join_strings(words):
    ret = result
    for word in words:
        ret += word
    return ret