Python Basics | Some Fundamentals in 10 minutes

1 | Introduction

1.1 | A simple start

print('Hello World')

Hello World

print('Hello! '*5)

Hello! Hello! Hello! Hello! Hello! 

1.2 | Python syntax is very readable and easily understood.

name = 'Batman'

if name == 'Batman':
    print('Hello Batman!')
else:
    print('Hello World!')

Hello Batman!

1.3 | The Python Intepreter

• The Python 'brain' is called the 'Python Interpreter' (Latest version is 3.9).
• The Python interpreter can only work on a single command at a time.
• If you give a list of commands, it goes through them one at a time.

print('Hello World!')                # Command 0
print('Hello Galaxy!')               # Command 1
print('Hello Universe!')             # Command 2

Hello World!
Hello Galaxy!
Hello Universe!

2 | Some Python Basics

2.1 | Comments in Python

• We can use a # to write comments.
• The Python interpretor ignores all comments.

print('One')              # This is a comment
# print('Two')            # The intepreter ignores this line 
print('Three')

One
Three

2.2 | Python is case sensitive

print('Hello World')      # This WILL work

Hello World

Print('Hello World')      # This will NOT work

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-40-e8337bbe3070> in <module>()
----> 1 Print('Hello World')      # This will NOT work

NameError: name 'Print' is not defined

2.3 | x is a variable. 'x' is an English letter.

print('x')      # Print the English letter 'x'

x

x = 10
print(x)        # Princt the value of the variable x

10

2.4 | = is not the same as '=='

• = is used to make something equal.
• == is used to check if something is equal.

x = 10            # Make x have the value 10

x == 20           # Tell me if x is equal to 20? 

False

x == 10           # Tell me if x is equal to 10? 

True

2.5 | Be careful of spaces(indentations) at the start of a line

# This WILL work
x = 10
print(x)

10

# This will NOT work
x = 10
    print(x)

  File "<ipython-input-47-53b0420fc6ee>", line 3
    print(x)
    ^
IndentationError: unexpected indent

2.6 | Python has many (many) cool packages

Basic Python is limited but can be extended with packages.

Let's understand what this means.

sqrt(4)       # Will NOT work because basic Python is limited

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-48-f00f1e21bca0> in <module>()
----> 1 sqrt(4)       # Will NOT work because basic Python is limited

NameError: name 'sqrt' is not defined

Let's import a package called math to fix this problem.

import math   # Adding(importing) the functions of the 'math' package

math.sqrt(4)

2.0

We can use other packages too. Lets consider the package numpy.

import numpy as np     # We are lazy! We type 'np' instead of 'numpy'

np.sqrt(4)

2.0

2.7 | The . and 'hidden' functions

The . shows ownership.

• math.sqrt() means the sqrt() function belonging to the package math
• np.sqrt() means the sqrt() function belonging to the package numpy
• 'Everything' in Python have useful functions that are sort of 'hidden'.

Example 1: upper() a method that belongs to 'letters'

'My name is Batman'.upper()

'MY NAME IS BATMAN'

Example 2: split() a method that belongs to 'letters'

'My name is Batman'.split()

['My', 'name', 'is', 'Batman']

Example 3: Available methods depends on what you have

---

1234.split()              # Will NOT work because split() is not a method available to numbers

  File "<ipython-input-55-d1af11342a47>", line 1
    1234.split()              # Will NOT work. split() is not a method available to numbers
             ^
SyntaxError: invalid syntax

2.8 | Python Lists and numpy Arrays

• Two import ways to store information in Python are Python Lists and numpy Arrays.

py_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]                  # A Python list
np_array = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10] )      # a numpy array

Althoug they look similary each have different features

Example 1: Selecting elements that satisfy a criterion

np_array[np_array > 4]    # Show me (subset) numbers more than 4

array([ 5,  6,  7,  8,  9, 10])

py_list[py_list > 4]      # Will NOT work                          

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-58-2e7235c251b1> in <module>()
----> 1 py_list[py_list > 4]      # Will NOT work

TypeError: '>' not supported between instances of 'list' and 'int'

Example 2: Adding a number to an array

np_array + 100            # Add 100 to all elements

array([101, 102, 103, 104, 105, 106, 107, 108, 109, 110])

py_list + 100             # Will NOT work  

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-60-7487b5debdc5> in <module>()
----> 1 py_list + 100             # Will NOT work

TypeError: can only concatenate list (not "int") to list

Example 3: Growing a list

py_list + [100,200]       # Appends the numbers to the list

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 200]

np_array + [100,200]     # Will NOT work  

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-62-b11ceebbfa9a> in <module>()
----> 1 np_array + [100,200]     # Will NOT work

ValueError: operands could not be broadcast together with shapes (10,) (2,) 

2.9| Accessing elements of lists and arrays

• Important: Python starts counting at 0

Examples 1: Selecting one element (indexing)

py_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 

# First item
py_list[0]

1

# Second item
py_list[1]

2

# Last item
py_list[-1]

10

Examples 2: Selecting a range of elements (slicing)

py_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 

# Start at index 3 (i.e. '4') and give two (5-3) elements
py_list[3:5]

[4, 5]

# Show me 3 elements from the start
py_list[:3]

[1, 2, 3]

# Show me all elements from the index 3
py_list[3:]

[4, 5, 6, 7, 8, 9, 10]

# Show me everything (from start to end)
py_list[:]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

2.10 | Getting Help

• Googling for help is the fastest way to get a problem solved.
• However, you can also get information from within Colab by using the following:

?np.cos