一个（可能的）Python作用域问题

Question

I cannot contribute my actual code (because it's over ten thousand lines), but I can give a brief summary of it. Here it is, I give the symptoms later:

def calculate_stuffs():
    *global lots of stuff*
    *some other stuff*
    for event in pygame.event.get():
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_BACKSLASH:
                try:
                    exec easygui.enterbox('Type command:')
                except:
                    print "Command crashed, or Easygui is not installed."
    *a lot of other stuff*

Here are some examples of when it works/fails:

• If I type a command that doesn't work, it returns the appropriate message.

• If I tell it to run a function, it works.

• If I tell it to read a variable (eg print a variable) it works.

• If I tell it to write to a variable, it doesn't do anything. It doesn't even give the crashing message. I made xa global variable, and it still does nothing if I say something like "x += 5".

I'm pretty sure this is an issue with scope, but I'm not sure.

If you need me to contribute more code, just ask. When you find the answer, please explain why it works. Thank you!

Answer 1

You need to use global within the exec statement as well, as exec has its own scope like a function:

x = 10

def foo():
    global x
    command = 'global x; x += 1; print x'
    try:
        exec command
    except:
        print "error"

print x
foo()
print x

will print:

10
11
11

Without global x within the exec statement it would print:

10
11
10