Python If ... Else

 

Python Condi tions and If statements

Python supports the usual logical conditions from mathematics:

• Equals: a == b
• Not Equals: a != b
• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in "if statements" and loops.

An "if statement" is written by using the if keyword.

Example

If statement:

a = 33
b = 200
if b > a:
  print("b is greater than a")

Output :

b is greater than a

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

If statement, without indentation (will raise an error):

a = 33
b = 200
if b > a:
print("b is greater than a") # you will get an error

Output:

b is greater than a

Elif

The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".

Example

a = 33
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")

Output :

a and b are equal

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".

---

Else

The else keyword catches anything which isn't caught by the preceding conditions.

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")
else:
  print("a is greater than b")

Output :

a is greater than b

In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif:

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
else:
  print("b is not greater than a")

Output:

b is not greater than a

Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

One line if statement:

a = 10

b = 2

if a > b: print("a is greater than b")

Output :

a is greater than b

Short Hand If ... Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

One line if else statement:

a = 2
b = 330
print("A") if a > b else print("B")

Output:

B

This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

One line if else statement, with 3 conditions:

a = 330
b = 330
print("A") if a > b else print("=") if a == b else print("B")

Output :

=

---

And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, AND if c is greater than a:

a = 200
b = 33
c = 500
if a > b and c > a:
  print("Both conditions are True")

Output :

Both conditions are True

---

Or

The or keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, OR if a is greater than c:

a = 200
b = 33
c = 500
if a > b or a > c:
  print("At least one of the conditions is True")

Output :

At least one of the conditions is True

Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

x = 41

if x > 10:
  print("Above ten,")
  if x > 20:
    print("and also above 20!")
  else:
    print("but not above 20.")

Output :

Above ten,

and also above 20!

---

The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

Example

a = 33
b = 200

if b > a:
  pass

The while Loop

 

Python Loops

Python has two primitive loop commands:

• while loops
• for loops

---

The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

Example

Print i as long as i is less than 6

i = 1
while i < 6:
  print(i)
  i += 1

Output : 

1

2

3

4

5

Note: remember to increment i, or else the loop will continue forever.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.

---

The break Statement

With the break statement we can stop the loop even if the while condition is true:

Example

Exit the loop when i is 3:

i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1

Output :

1

2

3

---

---

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

Example

Continue to the next iteration if i is 3:

i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)

Output :

1

2

4

5

6

---

The else Statement

With the else statement we can run a block of code once when the condition no longer is true:

Example

Print a message once the condition is false:

i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")

Output :

1

2

3

4

5

i is no longer less than 6

 

Import module in Python

Import in python is similar to #include header_file in C/C++. Python modules can get access to code from another module by importing the file/function using import. The import statement is the most common way of invoking the import machinery, but it is not the only way.

import module_name
When import is used, it searches for the module initially in the local scope by calling __import__() function. The value returned by the function are then reflected in the output of the initial code.

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import math print(math.pi)

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3.141592653589793

 

import module_name.member_name
In the above code module math is imported, and its variables can be accessed by considering it to be a class and pi as its object.
The value of pi is returned by __import__().
pi as whole can be imported into our intial code, rather than importing the whole module.

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from math import pi    # Note that in the above example,  # we used math.pi. Here we have used # pi directly. print(pi)

Output:

3.141592653589793

 

from module_name import *
In the above code module math is not imported, rather just pi has been imported as a variable.
All the functions and constants can be imported using *.

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from math import * print(pi) print(factorial(6))

Output:

3.141592653589793
720

As said above import uses __import__() to search for module, and if not found, it would raise ImportError

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import mathematics print(mathematics.pi)

Output:

Traceback (most recent call last):
  File "C:/Users/GFG/Tuples/xxx.py", line 1, in 
    import mathematics
ImportError: No module named 'mathematics'