5. Python’s basic syntax#

Author: Tue Nguyen

5.1. Outline#

Math operations
Assignments
Literals
Expressions
Comments
Printing
Statements
Line continuation
Whitespaces
Indentation
Get help

5.2. Overview#

This tutorial aims to provide just some very basic syntax so that you can get started immediately

It’s like when you first learn a new language

You don’t dive into learning grammar right away
Instead, you learn some basic vocabulary and simple sentences
This lecture serves such a purpose

In the next lectures, you will learn Python in a more systematic way

5.3. Math#

You can use Python as a calculator to compute math expressions
Math operations are intuitive. Ex: +, -, *, /

# Addition
2 + 3

# Subtraction
2 - 3

-1

# Multiplication
2 * 3

# Division
2 / 3

0.6666666666666666

# Integer division
2 // 3

# Modulus (get the remainder)
2 % 3

# Exponential (use `**`, not `^`)
2**3

# More complicated expressions
# Use parentheses if needed
1000 * (1 + 0.05)**20

2653.2977051444223

5.4. Assignments#

An assignment assigns a value to a variable (will learn in detail later)
We use = to make an assignment
After an assignment, we can use the variable name to refer to its associated value

# Assign a value 100 to variable x
x = 1000

# Use x as a reference to its associated value
x + 500

5.5. Literals#

A literal is a fixed or constant value
When you see it, it is literally the value it represents
No additional conversion is needed

# Integer literal
1000

# Boolean literal
True

True

# String literal
"This is a string"

'This is a string'

5.6. Expressions#

An expression is anything that can be evaluated as a value
It can be a combination of literals and variables, for example

# Add 2 integers
100 + 200

# Concatenate 2 strings
"Hello" + "World" 

'HelloWorld'

# Add an integer literal to a integer variable
x + 100

5.7. Comments#

Comments are used to document your code
Why? To help other people can understand what you are doing (if your code is not so obvious)
Other people also include future you because it is not uncommon that you will forget what you have done after a few weeks (or even days)
Comments are for human-only and are ignored by the interpreter
In Python, a comment starts with #

# This is a single-line comment

# This is also a comment
# but it spans
# on multiple lines

5.8. Printing#

You can use print() function to print out values associated with a literal, a variable, or an expression
Roughly speaking, a function (will learn in detail later)
- takes some input (inside the parentheses)
- does something with the input
- and spits out some output

# Print a number literal
print(1.25)

1.25

# Print a number literal using double quotes
print("Hello")

Hello

# Print a number literal using single quotes
print('Nice to meet you')

Nice to meet you

# Print a variable
total = 2 + 3
print(total)

# Print an expression
print(2**3 + 4)

5.9. Statements#

A statement is a complete instruction that Python can execute
It is like a full-sentence in English, for example

# An assignment statement
total = 2 + 3

# A print statement
print("Good morning")

Good morning

Warning

However, total = is not a statement
You will get an error if you try to run it.

Note 1: each statement normally sits on one line

print("Hello")
print("Good morning")

Hello
Good morning

Note 2: you can ends a statement with a semicolon

But it is not required
And it is not recommended as well (redudant + ugly)

# You can end a statement with ; 
# NOT recommended
print("Hello");
print("Good morning");

Hello
Good morning

Note 3: you can use semi-colins ; to write muliple statement on 1 line

But you should NOT do so
It make your code less readable

# Use ; to write multiple statements on 1 line
# NOT recommended
x = 5; y = 6; z = 7
print(x + y + z)

# A better alternative 
# write each statement on its own line
x = 5
y = 6
z = 7

print(x + y + z)

5.10. Line continuation#

On some occasions, you might have to write a complex statement
And keeping everything in one line make it very ugly and hard to read.
If so, you can break it into multiple (shorter) lines in two ways
- Method 1: put a backslash \ at the end of each line
- Method 2: wrap your code in a pair of parentheses

# Method 1: use \
x = 1 + 2 + 3 +\
    4 + 5 + 6 +\
    7 + 8 + 9

print(x)

# Method 2: use ()
x = (1 + 2 + 3 +
    4 + 5 + 6 +
    7 + 8 + 9)

print(x)

5.11. White spaces#

White spaces include spaces, tabs, newlines, and blank lines
Often, whitespaces are ignored by Python’s interpreter

Ex 1: extra spaces don’t count

print(2+3)
print(2 + 3)
print(    2 +     3)

5
5
5

Notes

All three statements print out the same result
However, the second line is considered to follow best practices
More on coding style guide in future lectures

Ex 2: new lines and blank lines don’t count either

# Snippet 1
x = 2 + 3

y = 4 + 5

print(x + y)

# Snippet 2
x = 2 + 3
y = 4 + 5
print(x + y)

Notes:

Again, snippet 2 is better than snippet 1 in terms of coding style

5.12. Indentation#

Indentation means the spaces at the beginning of each line
You use
- Tab to indent
- Shift + Tab to dedent
Unlike other languages, indentation does matter in Python
- Python uses indentation to indicate a code block
- It is similar to the use of curly brackets {} in other languages
For now, remember the following
- Never indent your code if you dont have a reason
- Always indent the new line after a statement that ends with : such as
  - An if statement
  - A for or while statement

a) Ex 1: try the following code (no error)

age = 30

if age >= 21:
    print("Congrats! You can buy vodka")

b) Ex 2: try the following code (error)

age = 30

if age >= 21:
print("Congrats! You can buy vodka")

5.13. Getting help#

You need help when you face things that
- You don’t know
- You know, but you forget the details
You can get help in 2 ways
- Internal documentation of Python using ?
- Google (tips: add “stackoverflow” to the search queries)

a) Ex 1: get internal help using ?

# Get help for print
?print

# Get help for abs

?abs

b) Ex 2: get help using Google

Try the following queries

Python compute absolute value stackoverflow
Python sort a list stackoverflow

5.14. Summary#

Math

Basic: +, -, * /
Modulus: //, %
Exponential: **
Use parentheses if needed

Assigment

Syntax: variable_name = value
After that, use the variable name to refer to the value

Literals

Literals = fixed values
Ex: 1000 (integer literal), True (Boolean literal), "hello" (string literal)

Expressions

An expression = anything that can be evaluated as a value
Can be a combination of literals and variables

Comments

Used for human and are ignored by Python
Start a comment with #
Comment can be on 1 line or multiple lines

Printing

Use print()
print() accepts a literal, variable, expression

Statements

A statement = a complete instruction that Python can execute
- total = 2 + 3 is a statement
- total = is NOT a statement
Some best practices
- Write each statement on 1 line
- Never end a statement with ;

Line continuation

Used to break a long statement into smaller pieces (for readability)
Two ways: use \ or ()

White spaces

White spaces = spaces, tabs, newlines, and blank lines
Often, whitespaces are ignored by Python’s interpreter
You whitespaces wisely to increase readability (more on next lectures)

Indentation

Indetation DOES matters
Keyboard
- Tab to indent
- Shift + Tab to dedent
For now
- Never indent your code if you have no reason
- Always indent the new line after : of if, for, and while statements

Getting help

Internal help: use ?
Online help: Google with the following template Python + key words + stackoverflow

Python tutorials

Python’s basic syntax

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