Python Syntax

Posted on October 6, 2017 by Administrator Posted in Computer Science, Python - Advanced, Python - Beginner, Python - Intermediate, Python Challenges

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

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Variables & Assignment Operator

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

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?"))

# 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.")

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))

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

# 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)

#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

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

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")

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")

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

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)

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!")

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

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()

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 = 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 = 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 = 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)

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!")

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")

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")

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")

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")

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")

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)

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.

Python Syntax

Annotations

Variables & Assignment Operator

Input

Output

Casting

String Manipulation

Random Library

Arithmetic Operators

Rounding a Number

IF Statements / Selection

Comparison Operators

Loops / Iteration

Lists

Procedures

Functions

File Handling – (Over)Writing a text file

File Handling – Appending data to a text file

File Handling – Reading a text file line by line

File Handling – Reading a CSV file line by line

File Handling – Reading a text file using a list

Input Validation: Presence Check

Input Validation: Type Check – Integer?

Input Validation: Range Check

Input Validation: Lookup Check

Input Validation: Character Check

Input Validation: Length Check

Try Again! Using a While Loop:

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