“I was what they call ‘skinny fat’ - a body that resembled a python after swallowing a goat.” - A. J. Jacobs

What is `Python`?

Python, one of the most popular programming languages in the world, has created everything from Netflix’s recommendation algorithm to the software that controls self-driving cars. Python is a general-purpose language, which means it’s designed to be used in a range of applications, including data science, software and web development, automation, and generally getting stuff done.

Python is a computer programming language often used to build websites and software, automate tasks, and conduct data analysis. Python is a general-purpose language, meaning it can be used to create a variety of different programs and isn’t specialized for any specific problems. This versatility, along with its beginner-friendliness, has made it one of the most-used programming languages today. A survey found that it was the most popular programming language among developers in 2020.

drawing

Figure source

Why `Python`?

Python is popular for a number of reasons. Here’s a deeper look at what makes it so versatile and easy to use for coders.

It has a simple syntax that mimics natural language, so it’s easier to read and understand. This makes it quicker to build projects, and faster to improve on them.
It’s versatile. Python can be used for many different tasks, from web development to machine learning.
It’s beginner-friendly, making it popular for entry-level coders.
It’s open source, which means it’s free to use and distribute, even for commercial purposes.
Python’s archive of modules and libraries, bundles of code that third-party users have created to expand Python’s capabilities, is vast and growing.
Python has a large and active community that contributes to Python’s pool of modules and libraries, and acts as a helpful resource for other programmers. The vast support community means that if coders run into a stumbling block, finding a solution is relatively easy; somebody is bound to have run into the same problem before.

drawing

Figure source

First look at `Spyder`

Follow instructions to install Spyder.

The Spyder IDE (Integrated Development Environment) is a simple and beginner-friendly integrated development environment for Python. It allows you to write, run, and debug your Python code.

drawing

Introduction to `Python`

The flow

By default, Python runs the script line by line. For example, if you run the following lines, a triangle would show up in your Console.

# Print a triangle
print("    /|")
print("   / |")
print("  /  |")
print(" /   |")
print("______")

Always think about clarifying your intentions, as others may later read your code. Here we call such intentions as “comments”. Anything that follows after the hash symbol # is ignored by Python when it executes code.

We can write a few lines with meaningful information:

# Print a few lines with meaningful information
print("Once there is a guy Lei, ")
print("he is 18 years old.")
print("He likes the name Lei,")
print("but does not like being 18.")

The output in your Console will be in the same order.

Variables

Suppose you want to change the name Lei and age 36 in the previous chunk of code, something like:

# Print a few lines with meaningful information
print("Once there is a guy Peng, ")
print("he is 22 years old.")
print("He likes the name Peng,")
print("but does not like being 22.")

It would be very handy if we could create variables to contain those pieces of information, and later use those variables directly.

# Create variables
name = "Lei"
age  = "18"

# Use variables
print("Once there is a guy " + name + ", ")
print("he is " + age + " years old.")
print("He likes the name " + name + ", ")
print("but does not like being " + age + ".")

Here you see we use = to assign the variables. A string is defined with the "", and we use + to combine two strings.

Now change the name and age to feel the power of variables:

# Create variables
name = "Superman"
age  = "180"

# Use variables
print("Once there is a guy " + name + ", ")
print("he is " + age + " years old.")
print("He likes the name " + name + ", ")
print("but does not like being " + age + ".")

Or use \n inside a string as ENTER to start a new line:

# Create a variable
school = "School of Environment"

# Use +
print(school + " is Cool!")

# Use \n
print("School of Environment \nis cool!")

Python variables are case-sensitive.

# Create variables
Age = "200"
age = "180"

print(Age)
print(age)

Working with strings

Python provides many built-in functions to handle strings, here are some commonly-used ones:

# Create a variable
school = "School of Environment"

# Change all characters into lower case
print(school.lower())

# Change all characters into upper case
print(school.upper())

# Are all characters in upper case? 
print(school.isupper())

# Get the length of the string
print(len(school))

Here we use . to call a function, and then apply the function to school. You may also use several functions one after another:

# Call upper() first, then pass the output to isupper()
print(school.upper().isupper())

If you want to get a specific character, use index. Remember, index in Python starts from 0.

# Print the 1st character
print(school[0])

# Print the 2nd character
print(school[1])

# Print a substring 
print(school[5:10])

Use index() to return the index of a string, or replace() to replace a string with something else.

# Use index()
# School of Environment
# 012345678901234567890
print(school.index("E"))

# Return the 1st match case
print(school.index("o"))

# Return the index of the substring
print(school.index("Env"))

# Try this, can you find z?
print(school.index("z"))

# Use replace()
print(school)
print(school.replace("Environment","Chemistry"))
print(school)

Working with numbers

When using Python as a calculator, the order of operations is the same as you would have learned back in school. From highest to lowest precedence:

Parentheses: (, )
Exponents: **
Multiply: *
Divide: /
Add: +
Subtract: -

# Get the square root of 100
100 ** 0.5

Really small or large numbers get a scientific notation:

2 / 100000

You can write numbers in scientific notation too:

5e+5 * 1e+5
5.2E+5 + 4.8E+6

To convert number to string, use str()

# Convert number to string
age = 18
print(str(age))
print(str(age) + " is my age")

# This would lead to an error
print(age + " is my age")

Mathematical functions

Python has many built-in mathematical functions. To call a function, we can type its name, followed by open and closing parentheses (). Anything we type inside the parentheses is called the function’s arguments:

# Get the mod
print(10 % 3)

# Get the absolute value
abs(-5)

# Get the power
pow(10, 2)

# Max and min
max(4, 6)
min(4, 6)

# Round
round(3.2)
round(3.7)

# Round with 1 digit
round(3.71,1)

You can find more mathematical functions in the math library. To import those functions, we type:

from math import *
floor(3.7)
ceil(3.7)
sqrt(36)

from math import * means we import all functions from the math library.

Find more functions and how to use them at Mathematical functions.

User inputs

Use input() to ask for inputs from the user:

# Ask for inputs from a user
name = input("Please enter your name: ")
age  = input("Please enter your age: ")

print("Hello " + name + "! You are " + age)

Comparing things

We can also do comparisons:

# Equality (note two equals signs, read as "is equal to")
1 == 1
# Inequality (read as "is not equal to")
3 != 2  
# Less than
100 < 101  
# Less than or equal to
1e3 <= 2e3
# Greater than
1/3 > 1/5
# Greater than or equal to
-100 > -200
# Use not() function
not(1 == 1)

Such True or False is called bool type. So far, we have covered int, float, string, and bool. You can always use type() function to determine the type of a variable.

# Use type() function
type(1)
type(1.0)
type("1")
type(1 == 1)

List

Python has a great built-in list type named list. List literals are written within square brackets [ ]. Lists work similarly to strings - use the len() function and square brackets [ ] to access data, with the first element at index 0.

# Make a list
my_list = ["Xiaohua", "Xiaoming", "Xiaoli", 9, 10.1]

# Print the list
print(my_list)

# Show the 1st element
print(my_list[0])

# Show the last element
print(my_list[-1])

# Includes element at index 2 and 3,
# excludes element at index 4
print(my_list[2:4])

# Show the 2nd element and afterward
print(my_list[1:])

Some common functions to handel lists:

# Make a list
firends = ["Xiaohua", "Xiaoming", "Xiaoli", "Xiaoyan", "Xiaoguang", "Xiaoping"]

# Make another list
ages    = [20, 31, 18, 29, 45, 15]

# Extend the list by appending all the items from the iterable
firends.extend(ages)
print(firends)

# Add an item to the end of the list
firends.append("99")
print(firends)

# Return index in the list of the first item whose value is equal to x
print(firends.index("Xiaoli"))
print(firends.index("Xiaoniu"))

# Insert an item at a given position
firends.insert(0,"Xiaoyong")
print(firends)

# Remove the first item from the list whose value is equal to x
firends.remove("Xiaoli")
print(firends)

# Remove all items from the list
firends.clear()
print(firends)

#------------------------------
# Make a new list
firends = ["Xiaohua", "Xiaoming", "Xiaoli", "Xiaoyan", "Xiaoguang", "Xiaoping", "Xiaoming"]

# Return the number of times x appears in the list
firends.count("Xiaoming")

# Sort the items of the list
firends.sort()
print(firends)

ages.sort()
print(ages)

# Reverse the elements of the list 
firends.reverse()
print(firends)

ages.reverse()
print(ages)

# Return a shallow copy of the list
firends2 = firends.copy()
print(firends2)

For more functions, please refer to Python Data Structures - More on List.

Tuple

A tuple is a collection of objects which are ordered and immutable. Tuples are sequences, just like lists. The differences between tuples and lists are, the tuples cannot be changed unlike lists; and tuples use parentheses (), whereas lists use square brackets [].

# Define a tuple
coordinates = (25.6, 119.8)

# Print the tuple
print(coordinates)

# Print the 2nd element
print(coordinates[1])

# Can you change it?
coordinates[1] = 120.7

# A more complex tuple
coordinates = [(25.6, 119.8), (22.9, 118.8), (21.7, 116.2)]

# Print the tuple
print(coordinates)

# Print the 2nd element
print(coordinates[1])

# What does this print?
print(coordinates[1][1])

For more functions, please refer to Python Data Structures - Tuples and Sequences.

Function

A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing.

As you already know, Python gives you many built-in functions like print(), etc., but you can also create your own functions. These functions are called user-defined functions.

Let’s define a simple function to say hi:

# Define a function to say hi
def say_hi():
    print("Hi there!")

Function blocks begin with the keyword def, followed by the function name and parentheses ( ). The code block within every function starts with a colon : and is indented - things not indented are not part of the function.

Call the function by:

# Call the function
say_hi()

We can pass a variable, aka parameter, in the function like:

# Define a function with one parameter
def say_hi(name):
    print("Hi " + name + "!")

say_hi("ESE5023ers")

You can pass more than one parameter:

# Define a function with two parameters
def say_hi(name, age):
    print("Hi " + name + ", you are " + age)

say_hi("Xiaoming", "20")

Return values from a function

The statement return exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

# Define a function to return the square of a number
def get_square(num):
    return num*num

get_square(10)

Or you can assign the return value into a variable:

result = get_square(50)
print(result)

Conditional statements

Often when we’re coding, we want to control the flow of our actions. This can be done by setting actions to occur only if a condition or a set of conditions are met.

There are several ways you can control flow in Python. For conditional statements, the most commonly used approaches are the if and else constructs.

Given today’s AQI (Air Quality Index) value, suppose we want to write a piece of code to check whether the Air Quality is excellent (AQI <= 50) or not.

AQI = 59
# If this condition is TRUE
if (AQI <= 50):
  # Do the following
  print("Air Quality is Excellent")

The print statement does not appear in the Console because AQI is larger than 50. To print a different message for numbers larger than 50, we can add an else statement.

# If this condition is TRUE
if (AQI <= 50):
  # Do the following
  print("Air Quality is Excellent")
# If this condition is FALSE  
else:
  print("Air Quality is NOT Excellent")

You can also test multiple conditions by using elif (means else if).

# Now use else if  
if (AQI <= 50):
  # Do the following line
  print("Air Quality is Excellent")
elif (AQI <= 100):
  # Do the following line
  print("Air Quality is GOOD")
else:
  # Do the following line
  print("Air Pollution!")

Change AQI to 40, 80, and 120, check the output.

Important: when Python evaluates the condition inside if statements, it is looking for a logical element (bool), i.e., TRUE or FALSE.

AQI1 = 69
AQI2 = 140

if (AQI1 <= 100 or AQI2 <= 100):
  print("There is at least 1 site with a GOOD air quality")

if (AQI1 <= 100 and AQI2 <= 100):
  print("Both sites have GOOD air quality")

Change AQI2 to 40, 80, and 120, check the output.

Seeking Help

Reading Help files

Python, and every library, provide help files for functions. The general syntax to search for help on any function, “function_name”, from a specific function that is in a library imported into your namespace (your interactive Python session):

help(function_name)

This will load up a help page.

When your code doesn’t work: seeking help from your peers

If you’re having trouble using a function, 9 times out of 10, the answers you are seeking have already been answered on Stack Overflow. You can search using the [python-3.x] tag. Finally, you will always find Google is your good friend.

In-class exercises

Exercise #1

Create a folder named ESE5023
- Windows: In C:\ or D:\ disk
- macOS: In /home/
Change Spyder Working directory to the above folder.

Exercise #2

X1  = 50
X2  = 120
X3  = X2 * 2.0
X4  = X1 - 20
X5  = X1 > X2

What will be the value of each variable after each statement in the program?

Now type the above lines in Console, check your results.
Write a command to compare X3 to X4. Which one is larger?
Clean up your variables by deleting the X1, X2, and X3.

Exercise #3

Build a basic calculator:

First, it asks a user to input two numbers.
Then it prints the addition of the two variables.

[Hint: you may find float() function useful to convert a string into a number]

Exercise #4

Create a function f2c that returns temperatures in degrees Celsius (C) with Fahrenheit temperature (F) as input parameter, based on the following equation: \[C = (F - 32) * 5/9\]
Check output with F=32 and F=100.
Can you create its inverse function c2f to convert temperatures in degrees Celsius (C) to Fahrenheit (F) ?

Section 03 Python: Basics (I)

What is `Python`?

Why `Python`?

First look at `Spyder`

Introduction to `Python`

The flow

Variables

Working with strings

Working with numbers

Mathematical functions

User inputs

Comparing things

List

Tuple

Function

Return values from a function

Conditional statements

Seeking Help

In-class exercises

Exercise #1

Exercise #2

Exercise #3

Exercise #4

Further reading

Section 03 Python: Basics (I)

What is Python?

Why Python?

First look at Spyder

Introduction to Python

The flow

Variables

Working with strings

Working with numbers

Mathematical functions

User inputs

Comparing things

List

Tuple

Function

Return values from a function

Conditional statements

Seeking Help

In-class exercises

Exercise #1

Exercise #2

Exercise #3

Exercise #4

Further reading

What is `Python`?

Why `Python`?

First look at `Spyder`

Introduction to `Python`