Introduction to Python#

BB1000#

KTH

layout: false

What is Python#

Guido-portrait-2014

Python is a scripting language written by Guido van Rossum

My original motivation for creating Python was the perceived need for a higher level language in the Amoeba [Operating Systems] project. I realized that the development of system administration utilities in C was taking too long. Moreover, doing these things in the Bourne shell wouldn’t work for a variety of reasons. … So, there was a need for a language that would bridge the gap between C and the shell.

Who uses Python#

From small scripts to large applications

print("Hello world!")

How to run Python#

Command-line#

Many Python programs are run from the computer’s command-line interface (CLI). In the CLI you type commands that the computer interprets and acts upon.

  • The CLI on a mac/linux is a terminal program that reads commands in the bash scripting language (or equivalent)

  • The CLI on windows is the console program known as CMD.

  • The CLI prompt is a character string printed at the beginning of each line when waiting for a command

In these slides we use the convention

  • $ is the bash prompt (mac/linux)

  • C> is the CMD prompt (windows)

Typing python in the CLI starts an interactive Python session accepting Python commands

  • >>> is the Python prompt

  • The Python interactive shell is also known as the REPL (read-evaluate-print-loop)

    • R read a Python expression

    • E evaluate the expression

    • P print the value of the expressions to the screen

    • L loop (start over)

$ python
>>> print("Hello world")
Hello world
>>>

  • Use case: a desktop calculator

$ python
>>> 10 + 10*0.25
12.5
>>> _  * 2
25.0

  • in this context the underscore _ means the value of the previous expression

Running Python scripts#

Assuming you have a file with Python instructions:

hello.py

../_images/hi.png

you run the script by giving it as a command-line argument to the python interpreter

$ python hello.py
Hello world
Goodbye

Creating Python scripts#

Text editors#

To enter code into files you need to use a text editor (not a word processor like Microsoft Word). A text editor is good for programming if it automatically colors special keywords for the programming language of that file. A simple editor that fulfills this is nano.

../_images/nano.png

Developers survey on most popular programming editor : normally lists editors like vim, emacs, atom, sublime. Spending time to learn one well is worth the investment.

IDE:s#

IDE = Integrated development environment

$ pip install mu-editor
$ mu-editor hello.py
../_images/mu.png

Visual Studio Code#

  • Microsoft open source project

../_images/vsc.png

Pycharm#

  • Community and Professional editions from JetBrains

../_images/pycharm.png

  • see https://www.jetbrains.com/student/

Notebooks#

  • work/develop in browser

  • mix documentation and code

$ jupyter notebook
* good for exploration/experimentation/demonstration * not good for writing large structured programs

Language fundamentals#

Python names#

  • Names in Python are what most languages refer to as variables, or variable names

  • To save the value of an object a name can be assigned to it

  • The assignment operator is =

  • Assignment is to bind a name to an object

>>> x = 8 * 9

  1. The right-hand side is evaluated

  2. An int object with value 72 is created in memory

  3. An association is created with this object and the name x

                ┏━━┓
                ┃ x┃
    ┌─┐   ┌─┐   ┡━━┩ 
    │8│ * │9│ → │72│
    └─┘   └─┘   └──┘

  • The name can be used in other expressions as an alias for that value

>>> print(x + x)
144

Python values and types#

Objects and expressions in Python have a value of some type. A type determines the range of possible values and operations that are defined for objects of that type.

Numerical types#

  • whole numbers (int): e.g. -1, 7, 2000

  • decimal numbers (float): 3.14, 1.0 -7.25

  • complex numbers (complex): 1j, 7+5j

  • logical (bool): True, False

Numerical operations#

  • + Addition

  • - Subtraction

  • * Multiplication

  • / Division (7 / 2 → 3.5)

  • // Integer floor division (7 // 2 → 3)

  • % Integer modulus (remainder) (7 % 2 → 1)

Logical operations#

  • and

    • True and True → True

    • True and False → False

    • False and False → False

  • or

    • True or True → True

    • True or False → True

    • False or False → False

  • not

    • not True → False

    • not False → True

  • equality

    • 1 == 1 → True

    • 1 == 1.0 → True

    • 1 != 1 → False

  • identity

    • 1 is 1 → True

    • 1 is 1.0 → False

  • relational

    • 1 < 1 → False

    • 1 >= 1.0 → True

Strings: str#

  • a string is a sequence of characters (unicode)

  • literal strings are written within quotation marks

  • single ' and double " quotation marks have the same status

  • three quotation marks limit strings that can span several lines

  • there is no single character type

>>> print("It's time")
It's time


>>> print('Our boss is "nice". 😀')
Our boss is "nice". 😀


>>> print("""Hello
... world""")
Hello
world

String operations#

  • Concatenation

>>> 'hello' + 'world'
'helloworld'

  • Duplication

>>> 'bye' * 2
'byebye'

Common methods#

>>> 'FORTRAN'.lower()
'fortran'


>>> 'c++'.upper()
'C++'


>>> 'jo'.capitalize()
'Jo'

Container types#

The most important container types are lists, tuples and dictionaries

Lists#

  • A list is a ordered sequence of elements

  • A literal list in code is defined by square brackets and comma-separated elements [0, 1, 2]

  • List can have members of different types

  • List members are referenced with [n] where n=0, 1, 2...

  • A list can be empty, []

>>> colours = ['hearts', 'spades', 'diamonds', 'clubs']
>>> values = [2, 3, 4, 5, 6, 7, 8, 9, 10, 'knight', 'queen', 'king', 'ace']
>>> colours[0]
'hearts'

Note that brackets are both used in the notation for a literal list and for getting a member from a collection (not only for lists)

>>> a_list = [1, 'a']
>>> a_member = a_list[0]

Tuples#

  • An immutable (unchangeable) sequence of objects

  • Similar to lists

  • () is the empty tuple

  • (1,) contains 1 element -note the comma

Handy packing and unpacking

>>> t = 1, 2 # packing
>>> x, y = t # unpacking
>>> x
1
>>> y
2
>>> x, y = y, x # swapping (packing and unpacking)
>>> x
2
>>> y
1

Dictionaries#

  • The dict type define sets of key-value pairs

  • Curly braces with comma-separates pairs define a literal dict

  • Each pair is separated by a colon :

  • The key can be any immutable (unchangeable) object

  • Very useful for complex structures

  • Efficient and highly optimized

empty =  {} # empty dict
newdict = {'a': 1, 'b': 2}

Program logic#

Repetition (iteration, looping)#

  • The for ... in statement is used repeat the same operation for all elements of a sequence

  • A loop variable will reference the elements of the sequence, one at a time

Looping over lists#

>>> for e in [1, 2, 3]: 
...    print(e)
1
2
3

Looping over strings#

>>> for c in 'hello':
...     print(c)
h
e
l
l
o

Looping over dictionaries#

>>> d = {'a': 1, 'b': 2}
>>> for k, v in d.items():
...    print(k, v)
a 1
b 2


>>> for k in d:
...    print(k, d[k])
a 1
b 2

Branching#

Conditional execution of code blocks depending on whether an expression evaluates to True or not:

>>> if True:
...     print("Yes")
... else:
...     print("No")
Yes


>>> i = j = 0
>>> if i > j:
...     print(i, " is larger than ", j)
... else:
...     print(i, "is smaller than or equal to", j)
0 is smaller than or equal to 0

Most object types have some truthiness. Empty lists in a logical context evaluate to False, non-empty to True

>>> if []:
...     print("Non-empty list")
... else:
...     print("Empty list")
Empty list

Program units#

Functions#

  • Functions are objects that can take some input and return some output. Functions are the primary way of grouping code into independent units, that can be tested and reused

  • Function definitions start def, a name, parentheses with or without arguments (comma-separated) and a colon. The body of the function is indented with respect to the def keyword. The last line of a function definition is normally a return statement and determines the value of a function call

>>> def square(x):
...    x2 = x * x
...    return x2

  • Functions are called with function name and an actual parameter.

>>> square(2)
4

  • Inside the function the formal parameter x becomes a reference to the actual parameter 2.

Modules#

  • a file with python source

    • name is the filename without the .py extension

  • import modules to reuse code

  • members of module referenced with dot notation module.member

Commonly used Python modules

  • sys

  • os

  • math

sys#

  • system modules

  • needed e.g. for arguments to a script

  • sys.argv is a list containing command-line arguments

  • The first element sys.argv [0] is the file name

import sys
infile = sys.argv[1]

os#

  • Interaction with operating system

  • Example: execute a unix command

    import os
    os.system('/bin/date')

math#

  • all basic elementary functions

  • fundamental constants

    import math
    print(math.sin(math.pi/2))

Tip#

Many use the math modules as a desktop calculator

$ python
>>> from math import *
>>> print(pi/2)
1.5707963267948966
>>>

Writing/using your own modules#

  • Suppose you have written file hello.py with function say_hello

#hello.py
def say_hello():
    return "Hello world!"

  • To access the same function in other code, import the module

>>> import hello
>>> message = hello.say_hello()
>>> print(message)
Hello world!

Sample code: multiplication table#

Version 1: if we only know print

print(1 * 7)
print(2 * 7)
print(3 * 7)

Version 2: extract the varying data to a variable

i = 1
print(i * 7)
i = 2
print(i * 7)
i = 3
print(i * 7)

Version 3: introduction the for loop, indented blocks

for i in (1, 2, 3, 4, 5, 6, 7, 8 , 9, 10):
    print(i * 7)

Version 4: the range function

n = 7
for i in range(1, 11):
    print(i * n)

Version 5: define as a function

def print_mult_table(n):
    for i in range(1, 11):
        print(i * n)

Version 6: call the function from the command line

import sys

def print_mult_table(n):
    for i in range(1, 11):
        print(i * n)

if __name__ == "__main__":
    print_mult_table(int(sys.argv[1]))

  • the system variable __name__ has the value "__main__" (double underscores) when the file is executed as a program

  • it has the value equal to the file name when it is imported as a module

  • command-line argments appear in the system variable sys.argv, a Python list which has the file name as the first member (of index zero)

A common pattern when a file is used both as an independent script and as a module imported by other programs

Summary#

  • Basic syntax - indentation

  • Basic built-in types

  • Scalar and container types

  • Variables

  • Functions

  • Modules

Reference#

Standard documentation#

  • https://docs.python.org/3

On-line books#

Online tutorials#

  • https://docs.python.org/3/tutorial/

  • https://realpython.com

  • https://dabeaz-course.github.io/practical-python

Youtube#

  • Variables in Python https://www.youtube.com/watch?v=_AEJHKGk9ns