Python fundamentals

Overview

To provide an overview of core Python functionality and programming techniques
Refresh your memory of Python syntax
This page only covers the basics and is by no means exhaustive.

Basic variable types

Ints: integers; e.g. a = 2
Floats: floating-point numbers with decimals; e.g. a = 2.0
Strings: collection of characters contained in single or double quotes; individual characters can be accessed using an index (starting at 0)

a = 2
b = 2.0
s = "hello"
print(s[0])

Use the int, float, and str functions to convert between types

a = 2.00
print(int(a))

Mathematical operations

Operation	Description	Example
+	Addition	5 + 3 = 8
-	Substraction	5 - 3 = 2
*	Multiplication	5 * 3 = 15
/	Division	5 / 3 = 1.666…
//	Floor division (round down to an integer)	5 // 3 = 1
%	Modulo (compute remainder)	5 % 3 = 2
**	Exponent	5 ** 3 = 125

Boolean operations

Operation	Description	Example	Value
==	Is equal?	1 == 2	False
!=	Is not equal?	1 != 2	True
<	Less than?	1 < 2	True
>	Greater than?	1 > 2	False
<=	Less than or equal to?	1 <= 2	True
>=	Greater than or equal to?	1 >= 2	False

Logical operations

Operation	Description	Example	Value
and	Are both true?	1 < 2 and 3 < 2	False
or	Is one true?	1 < 2 or 3 < 2	True
not	Negate the conditional	not(1 < 2)	False

Data structures

Type	Example	Characteristics
List	L = [1, 1.0, ‘one’]	Mutable, iterable, ordered
Tuple	t = (1, 1.0, ‘one’)	Immutable, iterable, ordered
Set	s = {1, 1.0, ‘one’}	Mutable, iterable, unordered, unique
Dictionary	d = {‘a’:1, ‘b’:2, ‘c’:3}	Mutable, iterable, ordered

Mutable: Can be modified
Immutable: Cannot be modified
Ordered: Elements can be accessed using an index or a key

Data structures continued

Use list, tuple, and set functions to convert between types
Elements in lists and tuples can be accessed using an integer index (starting at 0)
Elements in dictionaries are accessed using keys

# create a list and print the first value
L = [1, 2, 3]
print(L[0])

# create a dictionary of gravitational accelerations in m/s2
g = {'Earth': 9.8, 'Mars':3.7, 'Jupiter':25}
print(g['Earth'])

9.8

If statements

Used to make a decision in a program
Runs an indented block of code if a conditional statement is true

i = 20

if i < 10:
    print("Doing something because i < 10 and the code is indented")
    
print('Printing non-indented code for all values of i')

Printing non-indented code for all values of i

If-else statements

Creates two pathways, the choice depends on whether a condition is true or false

i = 20

if i < 10:
    print('Doing something because i < 10')
else:
    print('Doing something else i >= 10')

Doing something else i >= 10

If-else-elif statements

Creates multiple pathways, the choice depends on which condition is true

i = 20

if i < 10:
    print('Doing something because i < 10')
elif i > 10:
    print('Doing something else because i > 10')
else:
    print('Doing something different from the other two cases')

Doing something else because i > 10

For loops

For repeating code a fixed number of times

for e in collection:
    # run indented code

The indented code is run until e has taken on every value in collection (which is an iterable object like a list or tuple)

# print the numbers from 0 to 10
for i in range(11):
    print(i, end=" ")

0 1 2 3 4 5 6 7 8 9 10

# capitalise words in a list
L = ['red', 'blue', 'green']
for c in L:
    print(c.capitalize(), end=", ")

Red, Blue, Green,

While loops

For repeating code until a condition becomes false

while condition:
    # run indented code

While loops are useful when you don’t know how many times to repeat code
Beware of infinite loops!

# compute the square numbers that are smaller than 450
n = 1

while n**2 < 450:
    print(n**2, end=", ")
    n += 1

1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441,

Break and continue

break is used to terminate a loop
continue is used to skip an iteration in a loop

for i in range(10):
    print(i, end = " ")

0 1 2 3 4 5 6 7 8 9

for i in range(10):
    if i == 4:
        break
    print(i, end = " ")

0 1 2 3

for i in range(10):
    if i == 4:
        continue
    print(i, end = " ")

0 1 2 3 5 6 7 8 9

Functions

Functions are mini-programs based on a collection of code that has been given a name
Functions are defined using the def keyword
Function inputs are called arguments
The return keyword is used to output data from a function

# add two numbers a and b together
def my_sum(a, b):
    c = a + b
    return c

c = my_sum(3, 6)
print(c)

Scripts, modules, and packages

Modules are Python files (.py) that contain variables, functions, etc
Packages are folders (directories) that contain modules
Scripts are top-level Python files that import packages and modules
Scripts are run (e.g. in Spyder) not modules/packages

A typical file structure might look like this:

emat30008/
|--- main.py
|--- circle.py

where main.py is a script that imports the module circle.py

Importing modules and packages

The import keyword is used to load Python code from modules and packages
There are many ways to do this; see EMAT10007 notes for more details

# import the math package
import math

# print the variable pi from the math package
print(math.pi)

3.141592653589793

Summary

These slides covered core Python functionality

Operations, data types, control flow, loops, functions, modules and packages

Topics not covered but which you are expected to know:

Variable scope (local, global), keyword and default arguments, classes, file input and output