::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact

Scope and Ambiguous Assignments In Python

March 21st, 2011
tech, python  [html]
Consider the following two Python snippets:
    name='Mary'
    def print_name():
      print name
    print_name()
    print name
  
    name='Mary'
    def print_name():
      name='John'
    print_name()
    print name
  
The first will print 'Mary', twice. The second will print 'Mary' once. This happens because while python interprets reads as looking outside the current scope, writes can't be [1] anything but local. So the assignment to 'name' inside 'print_name' creates a new variable that disappears when the function exits.

So now consider:

    name='Mary'
    def print_name():
      print name
      name='John'
    print_name()
    print name
  
This code will generate an error:
    Traceback (most recent call last):
      File "tmp.py", line 5, in <module>
        print_name()
      File "tmp.py", line 3, in print_name
        print name
    UnboundLocalError: local variable 'name' referenced before assignment
  
The error is because within a function a variable must be either local or global. If it's local, the 'print name' is illegal because 'name' isn't defined yet. If it's global, the 'name="John"' is illegal because you can't assign outside your scope. So python chooses "local" and decides that the 'print name' line is invalid.

[1] well, you could use the 'global' keyword

Comment via: r/Python

Recent posts on blogs I like:

Incoming Gantz-Led Government to Invest in Israel’s Infrastructure

Israel’s incoming prime minister Benny Gantz unveiled an emergency government, to take power following an upcoming confidence vote in the Knesset. The last two MKs required to give Gantz a 61-59 majority, two members of Gantz’s own Blue and White Party wh…

via Pedestrian Observations April 1, 2020

Finding home in the time of coronavirus

Disclaimer: I’m going to say this once. Obviously, I am not happy about the coronavirus’s threat to public health or the economic toll it’s taking. I do not think the existence of this pandemic is good. Just so happens that social distancing and remote wo…

via Holly Elmore March 31, 2020

Massachusetts should shut down immediately

We’ve been running 30x fewer tests than other states, and have been extremely tardy in responding to the Biogen outbreak.

via benkuhn.net March 15, 2020

more     (via openring)

More Posts:


  ::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact