reading-notes

Python Scope

• Scope - how variables and names are looked up in code
  • visibility of variable within code
  • LEGB - Local, Enclosing, Global, Built in
  • defines area of program where you can unambiguously access the name of a variable, function, object, etc.
  • implemented as dictionaries called namespace
  • In Scope = have access too, Out of Scope = no access
• Global - available to all code
  • assign a name outside of all functions
• Local - only available to code within a local scope
  • assign a name in a function
  • created at function call not definition
• Enclosing - special scope for nested functions
  • inner scope would be local, outer scope would be enclosing.
• Built In - created/loaded when script is ran or open interactive session
  • contains - keywords, functions, exceptions, and other built in attributes
  • __builtins__ - names in builtins are always loaded in global scope with this name
• available anywhere in code, provided by python
• __main__ - turns main script into a module that holds the main programs execution
  • global scope
• Namespace - dictionaries that implement scope

import sys
sys.__dict__.keys()

• names at top level module are stored here

• python will return the first variable or function name that matches your search

• Global Statement - define a list of names to be treated as global

counter = 0  # A global name
>>> def update_counter():
...     global counter  # Declare counter as global
...     counter = counter + 1  # Successfully update the counter
...
>>> update_counter()
>>> counter
1
>>> update_counter()
>>> counter
2
>>> update_counter()
>>> counter
3

• nonlocal statement - accessed from inner functions but not assigned or updated

nonlocal my_var  # Try to use nonlocal in the global scope
  File "<stdin>", line 1
SyntaxError: nonlocal declaration not allowed at module level
>>> def func():
...     nonlocal var  # Try to use nonlocal in a local scope
...     print(var)
...
  File "<stdin>", line 2
SyntaxError: no binding for nonlocal 'var' found

• import - use import to bring seperate modules to your main module
• comprehension - compact way to process all or part of elements in collection or sequence
  • can be brackets [], or culry braces {} containing an expression, followed by one or more for clauses and zero or one if statements per for clause
• exceptions - variable holds reference to exception raised by try statement
• class and instance - defining a class creates a new local scope.
  • names assigned at top are local
  • created at execution
• globals() - returns reference to current global scope or namespace dict
• locals() - updates and returns dict that holds copy of current state of local scope or namespace
• vars() - builtin function that returns .dict attribute of module, class, instance or any other object with dict attribute
• dir() - can use without arguments to get list of names in current scope, with argument function will try to return a list of valid attributes for object

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