3.7. String Input

3.7.1. Rationale

  • input() always returns str

  • Good practice: add space at the end of prompt

  • Good practice: always .strip() text from user input

  • Good practice: always sanitize values from user prompt

3.7.2. Input Str

input() function argument is prompt text, which "invites" user to enter specific information. Note colon-space (": ") at the end. Space is needed to separate user input from prompt. Without it, user inputted text will be glued to your question.

>>> name = input('What is your name: ')  # User input: 'Mark Watney'
>>>
>>> print(name)
Mark Watney
>>>
>>> type(name)
<class 'str'>

3.7.3. Input Int

input() always returns a str. To get numeric value type casting to int is needed.

>>> age = input('What is your age: ')  # User input: 42
>>>
>>> print(age)
42
>>> type(age)
<class 'str'>
>>>
>>> age = int(age)
>>> print(age)
42
>>>
>>> type(age)
<class 'int'>

3.7.4. Input Float

Conversion to float handles decimals, which int does not support:

>>> age = input('What is your age: ')  # User input: 42.5
>>>
>>> age = int(age)
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: '42.5'
>>>
>>> age = float(age)
>>> print(age)
42.5
>>>
>>> type(age)
<class 'float'>

Conversion to float cannot handle comma (',') as a decimal separator:

>>> age = input('What is your age: ')  # User input: 42,5
>>>
>>> age = int(age)
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: '42,5'
>>>
>>> age = float(age)
Traceback (most recent call last):
ValueError: could not convert string to float: '42,5'
>>>
>>> float(age.replace(',', '.'))
42.5

3.7.5. Automated Input

  • Stub - simple object which always returns the same result

  • Mock - an object which has more advanced capabilities than Stub, such as:

    counting number of calls, recording passed arguments etc.

Note, that usage of Stubs and Mocks is only for testing purposes. You should not do that in your programs at the production level! Mocks and Stubs assumes, that user will input particular values.

3.7.6. Stub

You can also use Stub (a function with fixed value) to simulate input():

>>> def input(_):
...     return 'Mark Watney'
>>>
>>>
>>> input('What is your name?: ')
'Mark Watney'

At the end of the chapter about Functions, there is a mention about lambda expression. This could be also used here to make the code more compact.

>>> input = lambda _: 'Mark Watney'
>>>
>>>
>>> input('What is your name?: ')
'Mark Watney'

Both methods: Function Stub and Lambda Stub works the same.

3.7.7. Mock

In following example we simulate input() built-in function with MagicMock. Then, usage of input() is as normal.

>>> from unittest.mock import MagicMock
>>>
>>>
>>> input = MagicMock(side_effect=['Mark Watney'])
>>>
>>> input('What is your name?: ')
'Mark Watney'

Using MagicMock you can simulate more than one future input values from user:

>>> from unittest.mock import MagicMock
>>>
>>>
>>> input = MagicMock(side_effect=['red', 'green', 'blue'])
>>>
>>> input('Type color: ')
'red'
>>> input('Type color: ')
'green'
>>> input('Type color: ')
'blue'

Mocks has advantage over stubs, because they collect some diagnostic information.

>>> input.called
True
>>> input.call_count
3
>>> input.mock_calls  
[call('Type color: '),
 call('Type color: '),
 call('Type color: ')]

3.7.8. Assignments

Code 3.23. Solution
"""
* Assignment: String Input Prompt
* Required: yes
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Ask user to input text `NASA`
    2. Define `result: str` with text from user
    3. `MagicMock` will simulate inputting of `NASA` by user
    4. Use `input()` function as normal
    5. Run doctests - all must succeed

Polish:
    1. Poproś użytkownika o wprowadzenie tekstu `NASA`
    2. Zdefiniuj `result: str` z tekstem wprowadzonym od użytkownika
    3. `MagicMock` zasymuluje wpisanie `NASA` przez użytkownika
    4. Skorzytaj z funkcji `input()` tak jak normalnie
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> assert result is not Ellipsis, \
    'Assign result to variable: `result`'
    >>> assert type(result) is str, \
    'Variable `result` has invalid type, should be str'
    >>> assert input.call_count == 1, \
    'Call `input()` function'
    >>> assert 'NASA' in input.call_args.args[0], \
    'Ask user to input `NASA`'

    >>> result
    'NASA'
"""

from unittest.mock import MagicMock


# Simulate user input (for test automation)
input = MagicMock(side_effect=['NASA'])

# str: Ask user to type NASA
result = ...