Python samples

Blocks

In python blocks do not define scopes, however Functions, Objects, Modules do.

Conditional statements

Python does not have switch/case, instead use elif statement.

Loops I

Two loops; for & while.

Class introspection

These functions work by using Class introspection:

• type()
• id()
• isinstance()

Conditional Operators

1. Comparison Operators

   operator	py	desc
   ==	a == b	Equal
   !=	a != b	NotEqual
   <	a < b	Less than
   >	a > b	Greater than
   <=	a <= b	Less than or equal
   >=	a >= b	Greater than or equal

2. Logical Operators

   operator	py	desc
   and	x and y	True if both x and y
   or	x or y	True if x or y
   not	not x	Invert state

3. Identity Operator

   py	desc
   x is y	True if the same object
   x is not y	True if not the same object

4. Membership Operators

   py	desc
   x in y	True if x member of collection y
   x not in y	True if x not member of collection y

Arithmetic Operators

desc	operator
Addition	+
Subtraction	-
Multiplication	*
Division	/
Integer Division	//
Remainder(modulo)	%
Exponent	**
Unary negative	-
Unary positive	+

    # Unary operators
    z = -1
    z = +z
    print(z) # output: -1

Bitwise Operators

desc	operator
And	&
Or
Xor	^
Shift left	<<
Shift right	>>

Loops II

You can use else in while, or for loop!

    while pw != secret:
        if pw == '123': break
        pw = input('Enter secret word:')
    else: # in case while breaks, else won't run
        print('>Secret key is correct.')


    animals = ('bear', 'bunny', 'dog', 'cat')
    for pet in animals:
        print(pet)
    else:
        print('>Items finished.')

Functions

Arguments

To use argument lists, use *. For keyword arguments (a.k.a dictionary), use **

    ## Argument list
    def func(*args):
        if len(args):
            for arg in args:
                print(arg)

    args = ('one', 'two', 'three')
    func(*args)

    # Keyword arguments or dictionary arguments
    def kitten(**kwargs):
        if len(kwargs):
            for k in kwargs:
                print(f'Kitten {k} says {kwargs[k]}')

    x = dict(Buffy='meow', Zilla='grr', Angel='rawr')
    kitten(**x)

Generators

range() is an example of generators, it is useful for creating a series of values. It uses yield return.

    def inclusive_range(*args):
        ...
        i = start
        while i <= stop:
            yield i
            i += step

Decorators

    def f1(f):
        def f2():
            print('before function call')
            f()
            print('after function call')
        return f2

    @f1
    def f3():
        print('this is f3')

    f3()

Data structures

These collections may contain any object or type.

1. The list type: ordered collection, sequential, iteratable, mutable

        x = [1, 2, 3, 4, 5]

2. Tuple: exactly similar to list but the only difference: it is immutable

        x = (1, 2, 3, 4, 5)

3. The dictionary type: key-value pairs or hashed array. strings and numbers can always be keys. keys must be immutable.

        # two syntax
        x = {"a": 1, "b": 2, "c": 3}
        y = dict(a=1, b=2, c=3)

4. The set type: unordered list of unique values. It's exactly like a list without duplicate elements.

        # two syntax
        x = {1, 2, 3, 4, 5}
        y = set(12345)

   You can sort a set using sorted() function.

Classes

Special methods

Special methods start and ends with double underscores e.g. init

class Animal:
    # Class Variables, same for all objects of a class if they are mutable
    ## if you need a constant variable for your class, use immutable types
    x = 'I am a class variable'

    def __init__(self, **kwargs):
        # Object Variables starts with underscore _ this means do not touch this! use setters instead.
        # python doesn't have private variables
        self._type = kwargs['type'] if 'type' in kwargs else '-'
        self._name = kwargs['name'] if 'name' in kwargs else '-'
        self._sound = kwargs['sound'] if 'sound' in kwargs else '-'
    ...
    ## special method str, overriding string conversion
    def __str__(self):
        return f'Name: {self.name()}, Type: {self.type()}, Sound: {self.sound()}'
    ...

Generator vs Iterator

Check samples number 12 (using generator which uses yield return) and 18 (using Iterator).

• Generator function is often easier to implement
• Iterator is functionally identical to generator

Files

File endings:

1. LF(Line Feed): Linux-based OS file ending e.g. Mac, etc (1 byte)
   1. decimal: ASCII 10
   2. Hex: 0a
2. CR(Carriage Return): e.g. apple2, classic Macs (1 byte)
   1. decimal: ASCII 13
   2. Hex: 0d
3. CRLF: e.g. Windows, OS2, etc (2 bytes)

Two ways to write into a file in python:

1. using writeline()

       infile = open('02/23-lines.txt', 'rt')
       outfile = open('02/23-lines-copy.txt', 'wt')
       for line in infile:
           outfile.writelines(line)
           print('.', end='', flush=True)
       outfile.close()

2. using print(): with rstrip() you can rewrite file endings with the default os file endings

       infile = open('02/23-lines.txt', 'rt')
       outfile = open('02/23-lines-copy.txt', 'wt')
       for line in infile:
           # rstrip(): rewrite file endings with the default os file endings
           print(line.rstrip(), file=outfile)
           print('.', end='', flush=True)
       outfile.close()