Python Syntax

python-logo
This page summarises the syntax of the Python language. When using Python always remember two key rules:

  • Rule #1: Python is white-space dependent; code blocks are indented using spaces.
  • Rule #2: Python language is case sensitive. It matters for variables, functions and any keyword in general.
Input / OutputSelection (IF Statements)Iteration (Loops)ListsSubroutinesFile HandlingInput Validation

Annotations

# Single Line Comment

"""
Multi-line comment
by 101Computing.net
"""

Variables & Assignment Operator

myInteger = 1
myFloat = 3.14
myString = "Hello World"
myList = ["John","James","Laura"]

Input

# Text Input
playerName = input("What's your name?")

# Number/Integer Input
playerAge = int(input("How old are you?"))

# Number/Decimal Input
cost = float(input("How much does it cost?"))

Output

print("Hello World")

print("Hello " + playerName)

print("You are " + str(playerAge) + " years old.")

Casting

str(100)	# To convert a value to a string

int("100")	# To convert from a string to an integer

float("3.14")	# To convert from a string to a float

score = 5
print("Your score = " + str(score))

String Manipulation

# String Concatenation
playerName = "John"
myString = "Hello " + playerName + "!"

score = 5
print("Your score = " + str(score))


# Changing the case of a string
myString = "Hello World"
print(myString.upper())   # to print HELLO WORLD
print(myString.lower())   # to print hello world


# Extracting Characters of a String
myString = "Hello World"

print(myString[0])	# "H"
print(myString[0:5])	# "Hello"
print(myString[-5:])	# "World"


# Finding out the length of a string
print(len(myString))	# 11


# Splitting a string into a list of values
names = "John;James;Laura"
data = names.split(";") 
print(data[0]) # This would print: John

Random Library

#Import the random library
import random

#Generate a random number between 1 and 100
randomNumber = random.randint(1,100)

#Pick a value in a list at random
names = ["John","James","Luke"]
randomName = random.choice(names)

Arithmetic Operators

x = a + b	# Add
x = a – b	# Take away
x = a / b	# Divide
x = a * b	# Multiply
x = a ** 2	# to the power of
x = a // b    	# Quotient DIV
x = a % b   	# Remainder MOD

x += 1 # Increment x by 1
x -= 1 # Decrement x by 1
x *= 2 # Multiply x by 2
x /= 2 # Divide x by 2

Rounding a Number

pi = 3.14159
pi2 = round(pi,2)

print("Pi rounded to two decimal places: " + str(pi2))   # 3.14

IF Statements / Selection


Warning: Use the 4 spaces rule!!! (Indentation)

if numberOfLives == 0:
    print("Game Over!")


if number == 7:
    print("Seven")
else:
    print("Not seven!")


if number < 0:
    print("Negative Number")
elif number == 0:
    print("Zero")
else:
    print("Positive Number")


if number == 7:
    print("Seven")
elif number == 8:
    print("Eight")
elif number == 9:
    print("Nine")
else:
    print("Not seven, eight or nine")

Comparison Operators


The main comparison operators are:

Comparison Operator Meaning Example
== is equal to 4==4
!= is different from 4!=7
< is lower than 3<5
> is greater than 9>2
<= is lower or equal to 3<=5
5<=5
>= is greater or equal to 9>=2
5>=5

Comparison operators are used to check if a condition is true or false.

Boolean operators such as AND, OR and NOT can also be used to check multiple conditions at the same time.

e.g.

if timer>0 and numberOfLives>0:
    print("Carry on playing!")
else:
    print("Game Over")

Loops / Iteration


Warning: Use the 4 spaces rule!!! (Indentation)

for i in range(1,11):   # Using a range
    print i


for i in range(0,101,5):   # Using a step - e.g. count in 5
    print i

daysOfTheWeek = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]
#Iterate through all the values of a list
for i in range(0,len(daysOfTheWeek)):
    print(daysOfTheWeek[i])

#Iterate through all the values of a list - Alternative method
for day in daysOfTheWeek :
    print(day)


string = "Hello World"
for character in string:  # Using a string
    print(character)


i=0
while i<=100:
    print i
    i = i + 1

# To exit from a loop at any time:
    break

Lists

days = ["Mon","Tue","Wed","Thu","Fri","Sat"]

days.append("Sun")

print(days[0]) 	 # "Mon"
print(days[1]) 	 # "Tue"
print(days[2]) 	 # "Wed"
…
print(days[6]) 	 # "Sun"

#Finding out the number of items in a list (length of a list)
print(len(days)) # 7

#Iterate through all the values of a list
for i in range(0,len(days)):
    print(days[i])

#Iterate through all the values of a list - Alternative method
for value in days:
    print(value)

Procedures


Warning: Use the 4 spaces rule!!! (Indentation)

def displayBanner(message) :
    print("---------------")
    print(message)
    print("---------------")

displayBanner("Hello World!")

Functions


Warning: Use the 4 spaces rule!!! (Indentation)

def add(x, y) :
    sum = x + y
    return sum

total = add(1,3)
print(total)  # this would display 4

File Handling – (Over)Writing a text file

file = open("myTextFile.txt","w")

file.write("Hello World\n");
  
file.close()

Be careful, when using the write command, you are overwriting the content of your file. If instead of overwriting the content of your file you want to append (write at the end of the file) check the next tab: “Append to a text file”.

File Handling – Appending data to a text file

file = open("myTextFile.txt","a")

file.write("Hello World\n");
  
file.close()

File Handling – Reading a text file line by line

file = open("myTextFile.txt","r")

for line in file:
    print(line)
  
file.close()

File Handling – Reading a CSV file line by line


CSV files (Comma Separated Values) are text files that store data where each record is stored on a line and each field is separated using a divider symbol such as a comma “,”, a semi-colon “;” or a pipe “|”.

In this case when reading a line of the text file you can use the split() method to then access each field one by one.

file = open("myTextFile.txt","r")

for line in file:
    data = line.split(";")
    print(data[0] + " - " + data[1] + " - " + data[2])
  
file.close()

File Handling – Reading a text file using a list

file = open("myTextFile.txt","r")

lines = file.readlines()

file.close()

for line in lines:
  print(line)

Input Validation: Presence Check

name = input("Enter your name:")

if name=="":
    print("Empty name!")
else:
    print("Thank you!")

Input Validation: Type Check – Integer?

number = input("Type a number:")

if number.isdigit():
    print("This is a number")
else:
    print("This is not a whole number")

Input Validation: Range Check

number = int(input("Type a number between 1 and 5:"))

if number>=1 and number<=5:
    print("Valid number")
else:
    print("Invalid number")

Input Validation: Lookup Check

drive = input("Can you drive?").lower()

if drive in ["yes","no"]:
    print("Valid answer")
else:
    print("Invalid answer")

Input Validation: Character Check

email = input("Type your e-mail address:")

if "@" in email:
    print("Valid e-mail address")
else:
    print("Invalid e-mail address")

Input Validation: Length Check

password = input("Type a password:")

if len(password)>=8:
    print("Valid password")
else:
    print("Invalid password")

Try Again! Using a While Loop:

name = input("Enter your name:")

while name=="":
    print("You must enter your name! Please try again!")
    name = input("Enter your name:")

print("Welcome " +  name)

You can investigate more advance approaches to implement validation subroutines on this blog post.

Did you like this challenge?

Click on a star to rate it!

Average rating 4.2 / 5. Vote count: 306

No votes so far! Be the first to rate this post.

As you found this challenge interesting...

Follow us on social media!