Basic python programming¶

Running

Following the .gif below, click the rocket symbol () to launch this page as an interactive notebook in Google Colab (faster but requiring a Google account) or Binder.

Alt Text

In Google Colab, click Runtime -> Run all to run all the cells in the notebook. In Binder, click Cell -> Run All to run all the cells in the notebook.

Variable and expressions¶

Creating variables¶

In Python, variables are names to store values.
The = symbol assigns the value on the right to the name on the left.
The variable is created when a value is assigned to it.
Variable Naming (click to learn more about Python naming convention)
- can only contain letters, digits, and underscore _ (typically used to separate words in long variable names)
- cannot start with a digit
- are case sensitive (age, Age and AGE are three different variables)

Try the following code in the cell below to assign 1 to a variable x and "hello world" to a variable string_varible:

x = 1
string_variable = "Hello world"

Note

Variables must be created before they are used. If a variable doesn’t exist yet, or if the name has been mis-spelled, Python reports an error.

Warning

Some variable names are reserved for special use in Python. For example, print is a function that prints the value of a variable. If you try to use print as a variable name, Python will report an error.

Run the following code to see the full list of reserved words in Python:

import keyword

keyword.kwlist

['False',
 'None',
 'True',
 'and',
 'as',
 'assert',
 'async',
 'await',
 'break',
 'class',
 'continue',
 'def',
 'del',
 'elif',
 'else',
 'except',
 'finally',
 'for',
 'from',
 'global',
 'if',
 'import',
 'in',
 'is',
 'lambda',
 'nonlocal',
 'not',
 'or',
 'pass',
 'raise',
 'return',
 'try',
 'while',
 'with',
 'yield']

For loop and if statement¶

for loop is used to iterate over a sequence (e.g. a string) or other iterable objects.

Example

Display the characters in the string "Hello world":

for character in string_variable:
    print(character)

H
e
l
l
o
 
w
o
r
l
d

if statement is used to execute a block of code if a specified condition is true.
elif is used to check another condition if the first condition is false.
else is used to execute a block of code if all conditions are false.

Examples

if … else statement

# check an input number is non-negative or negative

num = 3

# Try these two variations as well.
# num = -5
# num = 0

if num >= 0:
    print("{} is a non-negative number".format(num))
else:
    print("{} is a negative number".format(num))

3 is a non-negative number

if … elif … else statement

# check an input number is positive, negative, or zero

num = 3.14

# Try these two variations as well:
# num = 0
# num = -2.5

if num > 0:
    print("{} is a positive number".format(num))
elif num == 0:
    print("The input number is zero")
else:
    print("{} is a negative number".format(num))

3.14 is a positive number

Built-in functions¶

There are many built-in functions in Python. For example, the print function has been used in the above section. This section introduces some common built-in functions:

Print

print displays the value of an expression.
Provide values to the function (i.e., the things to print) in parentheses.

print("Value of the string variable is ", string_variable)
print("The first character of the string variable is ", string_variable[0])
print("The first five characters of the string variable are ", string_variable[:5])
print(x, "+ 1 =", x + 1)

Value of the string variable is  Hello world
The first character of the string variable is  H
The first five characters of the string variable are  Hello
1 + 1 = 2

Type

The type function returns the type of an expression.

type(string_variable)

str

type(x)

int

Length

The len function returns the length of a string, or the number of elements in other type of variables, such as a list and tuple.

print(len(string_variable))

Range

The range function returns a sequence of numbers.

for i in range(3):
    print("loop: ", i)

loop:  0
loop:  1
loop:  2

Variable types¶

Numbers
- Integers (e.g. 1, 2, 3) and floating point numbers (e.g. 1.0, 2.5, 3.14159) are the two main numeric types in Python.

y = 1
z = 3.14
print(type(y))
print(type(z))

<class 'int'>
<class 'float'>

Strings
- Strings are sequences of characters.
- Strings are created by enclosing characters in single quotes ('...') or double quotes ("...").
- Strings can be concatenated (glued together) with the + operator, and repeated with *.

print(string_variable + " " + "Python is fun!")

Hello world Python is fun!

print(3 * string_variable)

Hello worldHello worldHello world

Booleans
- Booleans are either True or False.
- Booleans are often used in if statements to control the flow of a program, or used in while or for loops to control the number of times a loop is executed.

create_int_variable = True
if create_int_variable:
    new_int_variable = 123

print(new_int_variable)

None
- None is a special value that represents the absence of a value.
- None is the only value of the type NoneType.
- None is frequently used to represent the absence of a value, as when default arguments are not passed to a function.
- None is also frequently returned by functions that don’t explicitly return anything in order to explicitly signal the absence of a return value.
- None is a singleton object, there is only one None object and it is unique.
- None is immutable, it cannot be changed in any way.
- None is comparable to any other object using the is operator, but it is never equal to any other object using the == operator.

none_variable = None
print(none_variable is None)

print(type(none_variable))

True
<class 'NoneType'>

Lists
- Lists are ordered sequences of values.
- Lists are created by enclosing values in square brackets ([...]).
- Lists can contain values of different types.
- Lists can be indexed, sliced, and nested.
- Lists are mutable and dynamic.

new_list = [1, 2, 3, 4, 5, None]

Using the append() method can append an element to the end of the list.

new_list.append("Hello world")
print(new_list)

[1, 2, 3, 4, 5, None, 'Hello world']

Dictionaries
- Dictionaries are unordered sets of key: value pairs, and created by enclosing pairs in curly braces ({...}).
- Dictionaries can contain values of different types.
- Dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys.
- Dictionaries are mutable and dynamic.
- Dictionaries have no concept of order among elements.
- Dictionaries are sometimes found in other programming languages as “associative memories”, “associative arrays”, “associative lists”,“hashes”, “hash tables”, or “maps”.

Tuples
- Tuples are ordered sequences of values.
- Tuples are created by enclosing values in parentheses ((...)).
- Tuples can contain values of different types.
- Tuples can be indexed and sliced.
- Tuples are immutable and dynamic.

In this course, tuples are usually used to represent the shapes of vectors and matrices.

new_tuple = (3, 4)

type(new_tuple)

tuple

Type conversion
- Python can convert values from one type to another.
- This is called type conversion, and is sometimes also called type casting.
- The syntax for type conversion is to use the type name as a function.
- For example, int("32") converts the string 32 to an integer, and float(32) converts the integer 32 to a floating-point number.
- Type conversion can also be done with the built-in functions str(), int(), and float().

a = 1

print(a, type(a))

print(float(a), type(float(a)))

print(str(a), type(str(a)))

<class 'int'>
0 <class 'float'>
<class 'str'>

Indexing and slicing¶

Indexing is used to access a single element of a sequence (e.g. a string, a list, or a tuple).

Each position in the string (first, second, etc.) is given a number. This number is called an index or sometimes a subscript.
Indices are numbered from 0.
Use the position’s index in square brackets to get the character at that position.

print(string_variable[0])

Index value can be negative, which counts from the right. For example, the index value -1 refers to the last character in the string, -2 refers to the second-last character, and so on. See the following example to get the last element of a list:

print(new_list[-1])

Hello world

Note

In python, the index starts from 0, not 1.

Slicing is used to access a subsequence of a sequence.

A part of a string is called a substring. A substring can be as short as a single character.
An item in a list is called an element. Whenever we treat a string as if it were a list, the string’s elements are its individual characters.
A slice is a part of a string (or, more generally, a part of any list-like thing).
We take a slice with the notation [start:stop], where start is the integer index of the first element we want and stop is the integer index of the element just after the last element we want.
The difference between stop and start is the slice’s length.
Taking a slice does not change the contents of the original string. Instead, taking a slice returns a copy of part of the original string.

# elements beginning to index 5 (not included)
string_variable[:5]

'Hello'

# elements from index 3 to 5 (not included)
string_variable[3:5]

'lo'

# elements from index index 6 to end
string_variable[6:]

'world'

Calculations¶

Variables can be used in calculations as if they were values

x + 1

Exercises¶

1. Create a list with values \(22\), \(55\), \(30\), \(126\), \(198\), and \(225\). Write code to only print the numbers which are divisible by \(5\).

# Write your code below to answer the question

Compare your answer with the reference solution below

list_num = [22, 55, 30, 126, 198, 225]
for num in list_num:
    if num % 5 == 0:
        print(num)

55
30
225

2. Add the values \(33\) and \(65\) to the list and print all the list values.

# Write your code below to answer the question

Compare your answer with the reference solution below

list_num.append(33)
list_num.append(65)
for num in list_num:
    print(num)

3.

    a = 13  
    b = 4.76  
    c = 'TransparentML'  
    d = (2,5,9,14)   
    e = [3,4,2]    

Find out each variable types.

# Write your code below to answer the question

Compare your answer with the reference solution below

a = 13
b = 4.76
c = "TransparentML"
d = (2, 5, 9, 14)
e = [3, 4, 2]

print(a, type(a))
print(b, type(b))
print(c, type(c))
print(d, type(d))
print(e, type(e))

13 <class 'int'>
4.76 <class 'float'>
TransparentML <class 'str'>
(2, 5, 9, 14) <class 'tuple'>
[3, 4, 2] <class 'list'>

4. Print the following pattern using loop.

# Write your code below to answer the question

Compare your answer with the reference solution below

for i in range(6):
    for j in range(i):
        print(i, end=" ")
    print("")

Transparent ML Intro

Basic python programming

Contents

Basic python programming¶

Variable and expressions¶

Creating variables¶

For loop and if statement¶

Built-in functions¶

Variable types¶

Indexing and slicing¶

Calculations¶

Exercises¶