11.5. Function Scope

  • Values defined in function does not leak out

  • Functions has access to global values

  • Shadowing is when you define variable with name identical to the one from outer scope

  • Shadowing in a function is valid only in a function

  • After function return, the original value of a shadowed variable is restored

  • global keyword allows modification of global variable

  • Using global keyword is considered as a bad practice

11.5.1. Values Leaking

  • Values defined in function does not leak out

>>> def run(a, b=1):
...     c = 2
>>>
>>>
>>> print(a)
Traceback (most recent call last):
NameError: name 'a' is not defined
>>>
>>> print(b)
Traceback (most recent call last):
NameError: name 'b' is not defined
>>>
>>> print(c)
Traceback (most recent call last):
NameError: name 'c' is not defined
>>>
>>> run(0)
>>>
>>> print(a)
Traceback (most recent call last):
NameError: name 'a' is not defined
>>>
>>> print(b)
Traceback (most recent call last):
NameError: name 'b' is not defined
>>>
>>> print(c)
Traceback (most recent call last):
NameError: name 'c' is not defined

11.5.2. Outer Scope

  • Functions has access to global values

>>> data = [1, 2, 3]
>>>
>>>
>>> def run():
...     print(data)
>>>
>>>
>>> print(data)
[1, 2, 3]
>>>
>>> run()
[1, 2, 3]
>>>
>>> print(data)
[1, 2, 3]

11.5.3. Shadowing

  • When variable in function has the same name as in outer scope

  • Shadowing in a function is valid only in a function

  • Shadowed variable will be deleted upon function return

  • After function return, the original value of a shadowed variable is restored

>>> data = [1, 2, 3]
>>>
>>>
>>> def run():
...     data = [10, 20, 30]  # Shadows name 'data' from outer scope # noqa
...     print(data)
>>>
>>>
>>> print(data)
[1, 2, 3]
>>>
>>> run()
[10, 20, 30]
>>>
>>> print(data)
[1, 2, 3]

11.5.4. Global

  • global keyword allows modification of global variable

  • Using global keyword is considered as a bad practice

>>> data = [1, 2, 3]
>>>
>>>
>>> def run():
...     global data
...     data = [10, 20, 30]
...     print(data)
>>>
>>>
>>> print(data)
[1, 2, 3]
>>>
>>> run()
[10, 20, 30]
>>>
>>> print(data)
[10, 20, 30]

11.5.5. Global Scope

>>> globals()   
{'__name__': '__main__',
 '__doc__': None,
 '__package__': None,
 '__loader__': <class '_frozen_importlib.BuiltinImporter'>,
 '__spec__': None,
 '__annotations__': {},
 '__builtins__': <module 'builtins' (built-in)>}
>>> firstname = 'Mark'
>>> lastname = 'Watney'
>>>
>>> globals()  
{'__name__': '__main__',
 '__doc__': None,
 '__package__': None,
 '__loader__': <class '_frozen_importlib.BuiltinImporter'>,
 '__spec__': None,
 '__annotations__': {},
 '__builtins__': <module 'builtins' (built-in)>,
 'firstname': 'Mark',
 'lastname': 'Watney'}

11.5.6. Local Scope

  • Variables defined inside function

  • Variables are not available from outside

  • If outside the function, will return the same as globals()

>>> locals()  
{'__name__': '__main__',
 '__doc__': None,
 '__package__': None,
 '__loader__': <class '_frozen_importlib.BuiltinImporter'>,
 '__spec__': None,
 '__annotations__': {},
 '__builtins__': <module 'builtins' (built-in)>}
>>> def echo():
...     a = 1
...     print(locals())
>>>
>>>
>>> echo()
{'a': 1}
>>> def echo(a, b=2):
...     c = 3
...     print(locals())
>>>
>>>
>>> echo(1)
{'a': 1, 'b': 2, 'c': 3}

If outside the function, will return the same as globals():

>>> locals() == globals()
True

11.5.7. Shadowing Global Scope

  • Defining variable with the same name as in outer scope

  • Shadowed variable will be deleted upon function return

Shadowing of a global scope is used frequently in Mocks and Stubs. This way, we can simulate user input. Note that Mocks and Stubs will stay until the end of a program.

>>> def input(__prompt):  # noqa
...     return 'Mark Watney'
>>>
>>>
>>> name = input('Type your name: ')
>>> name
'Mark Watney'
>>>
>>> age = input('Type your age: ')
>>> age
'Mark Watney'
>>> from unittest.mock import MagicMock
>>> input = MagicMock(side_effect=['Mark Watney', '44'])  # noqa
>>>
>>>
>>> name = input('Type your name: ')
>>> name
'Mark Watney'
>>>
>>> age = input('Type your age: ')
>>> age
'44'

To restore default behavior of input() function use:

>>> from builtins import input

11.5.8. Builtins

>>> import builtins
>>>
>>> dir(builtins)   
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',
 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning',
 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError',
 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning',
 'EOFError', 'Ellipsis', 'EncodingWarning', 'EnvironmentError', 'Exception',
 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError',
 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning',
 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError',
 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError',
 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError',
 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError',
 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError',
 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError',
 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError',
 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError',
 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError',
 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError',
 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning',
 'ZeroDivisionError', '_', '__build_class__', '__debug__', '__doc__',
 '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs',
 'aiter', 'all', 'anext', 'any', 'ascii', 'bin', 'bool', 'breakpoint',
 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile',
 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod',
 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format',
 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id',
 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license',
 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object',
 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr',
 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod',
 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

11.5.9. Use Case - 0x01

>>> from unittest.mock import MagicMock
>>>
>>> # Simulate user input (for test automation)
>>> input = MagicMock(side_effect=['lastname'])  # noqa
>>>
>>>
>>> firstname = 'Mark'
>>> lastname = 'Watney'
>>>
>>> what = input('Type variable name: ')   #input: 'lastname'
>>>
>>> globals()[what]
'Watney'

11.5.10. Assignments

Code 11.18. Solution
"""
* Assignment: Function Scope Global
* Required: yes
* Complexity: easy
* Lines of code: 5 lines
* Time: 8 min

English:
    1. Define in global scope `SELECT: set[str]` with values:
       `'setosa', 'versicolor'`
    2. Define function `sumif(features, label)`
    3. Function sums `features`, only when `label` is in `SELECT`
    4. When `label` is not in `select` return `0` (zero)
    5. Run doctests - all must succeed

Polish:
    1. Zdefiniuj w przestrzeni globalnej `SELECT: set[str]` z wartościami:
       `'setosa', 'versicolor'`
    2. Zdefiniuj funkcję `sumif(features, label)`
    3. Funkcja sumuje `features`, tylko gdy `label` jest w `SELECT`
    4. Gdy `label` nie występuje w `select` zwróć `0` (zero)
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isfunction

    >>> assert isfunction(sumif), \
    'Object `sumif` must be a function'
    >>> assert SELECT is not Ellipsis, \
    'Assign result to variable: `SELECT`'
    >>> assert type(SELECT) is set, \
    'Variable `result` has invalid type, should be set'
    >>> assert all(type(x) is str for x in SELECT), \
    'All rows in `result` should be str'

    >>> sum(sumif(X,y) for *X, y in DATA[1:])
    49.1
"""

DATA = [
    ('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species'),
    (5.8, 2.7, 5.1, 1.9, 'virginica'),
    (5.1, 3.5, 1.4, 0.2, 'setosa'),
    (5.7, 2.8, 4.1, 1.3, 'versicolor'),
    (6.3, 2.9, 5.6, 1.8, 'virginica'),
    (6.4, 3.2, 4.5, 1.5, 'versicolor'),
    (4.7, 3.2, 1.3, 0.2, 'setosa'),
]