Python如何打印超出范围的变量

Question

I have the following function in Python that seems to be working:

def test(self):
    x = -1
    # why don't I need to initialize y = 0 here?
    if (x < 0):
        y = 23

    return y

But for this to work why don't I need to initialize variable y? I thought Python had block scope so how is this possible?

Answer 1

This appears to be a simple misunderstanding about scope in Python . Conditional statements don't create a scope. The name y is in the local scope inside the function, because of this statement which is present in the syntax tree:

y = 23

This is determined at function definition time, when the function is parsed. The fact that the name y might be used whilst unbound at runtime is irrelevant.

Here's a simpler example highlighting the same issue:

>>> def foo():
...     return y
...     y = 23
... 
>>> def bar():
...     return y
... 
>>> foo.func_code.co_varnames
('y',)
>>> bar.func_code.co_varnames
()
>>> foo()
# UnboundLocalError: local variable 'y' referenced before assignment
>>> bar()
# NameError: global name 'y' is not defined

Answer 2

It seems that you misunderstood this part of Python's documentation :

A Python program is constructed from code blocks. A block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition.
...
A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block.

So in this case block is something completely different from visual blocks of your code. Thereby if , for , while statements doesn't have their own scopes. But it is worth noting that comprehensions and generator expressions are implemented using a function scope , so they have their own scopes.

Answer 3

There is actually no block scope in python. Variables may be local (inside of a function) or global (same for the whole scope of the program).

Once you've defined the variable y inside the 'if' block its value is kept for this specific function until you specifically delete it using the 'del' command, or the function exits. From the moment y is defined in the function, it is a local variable of this function.

Answer 4

As in What's the scope of a Python variable declared in an if statement? : "Python variables are scoped to the innermost function or module; control blocks like if and while blocks don't count."

Also useful: Short Description of the Scoping Rules?