python/reference/exercise-concepts/variable-length-quantity.md

Concepts of variable-length-quantity

Example implementation

From the current example.py:

EIGHTBITMASK = 0x80
SEVENBITSMASK = 0x7f


def encode_single(n):
    bytes_ = [n & SEVENBITSMASK]
    n >>= 7

    while n > 0:
        bytes_.append(n & SEVENBITSMASK | EIGHTBITMASK)
        n >>= 7

    return bytes_[::-1]


def encode(numbers):
    return sum((encode_single(n) for n in numbers), [])


def decode(bytes_):
    values = []
    n = 0

    for i, byte in enumerate(bytes_):
        n <<= 7
        n += (byte & SEVENBITSMASK)

        if byte & EIGHTBITMASK == 0:
            values.append(n)
            n = 0
        elif i == len(bytes_) - 1:
            raise ValueError('incomplete byte sequence')

    return values

Concepts

• [Loops][loops]: the knowledge of while and for loops are useful to solve this exercise
• [Comparison][comparison]: concept required to solve the exercise, ==, >, <
• [Builtin Functions][builtin-functions]: the sum() built-in function is a useful concept to solve the exercise
• [Lists][lists]: knowledge of lists and iteration on lists is required for this exercise
• [Builtin Functions][builtin-functions]: the len() built-in function is a useful concept in this exercise
• [Conditionals][conditionals]: knowledge of if...else statements is required
• [List Methods][list-methods]: the use of methods of list could be useful in this exercise. Methods like append, pop...
• [Parameters][parameters]: concept over arguments of a function and how to use them is required
• [Return Value][return-value]: the knowledge of return statement could be a useful concept in this exercise
• [List Comprehensions][list-comprehensions]: the use of list comprehension can be useful in this exercise
• [Binary Numbers][binary-numbers]: binary numbers are a core important concept for this exercise
• [Bitwise Operators][bitwise-operators]: bitwise operators such as <<, >>, &, |, ~, ^ are central to this exercise
• [Iteration][iteration]: the for ... in concept is useful to loop over the lists
• [Builtin Functions][builtin-functions]: the enumerate built-in function is a useful concept in this exercise
• [Raise][raise]: the user could find useful the Exception concept to raise a ValueError for incorrect input
• [Exception Message][exception-message]: the user can use a custom error message inside the mentioned ValueError
• [Inheritance][inheritance]: the knowledge of inheritance can be useful in this exercises because all Exceptions types inherit from the base class