Python If ... Else


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Python Conditions and If statements

The common logical conditions from mathematics is supported by Python :

  • 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

Those conditions are most common in 'if statements' and loops to be used in various ways.


The IF Statement

The if keyword is used to executes a single block of code, if given conditions are true.

Example :- Simple If statement :

a = 33
b = 200

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

Output :-

b is greater than a

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

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Indentation

In order to define the scope, Python relies on indentation (whole space at the start of a line). For this purpose, other languages frequently use curly-brackets.

Example :- If an indentation is not provided (an error will be raised) :

a = 33
b = 200

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

Output :-

File "demo_if_error.py", line 4
print("b is greater than a")
^
IndentationError: expected an indented block

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Elif

The elif keyword is used to executes a single code block from multiple block of codes.

The keyword of elif is pythonic: "Try this condition if previous conditions have not been met".

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

a is equivalent to b, so the first condition is not true but the elif condition is true and therefore we print to screen the following: "a and b are the same".

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Else

Anything else that is not covered by the conditions above can be found.

Example 1 :-

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

a is greater than b, in this example, so the initial condition is not true, also the condition of the elif is not true, so we take the else condition and print to screen "a is greater than b".

Example 2 :- Without the elif, you can have else :

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

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Short Hand If

You can place it on the same line as if you only have one statement to perform.

Example 1 :- One line if statement :

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

Output :-

"a is greater than b"

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Short Hand If ... Else

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

Example 1 :- One line if else statement :

a = 2
b = 330

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

Output :-

B

This technique is referred to as Ternary operators, or conditional statements.

On the same line you can also make several else statements :

Example 2 :- 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 :-

=

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And

The keyword and the logical operator is used to combine the conditions :

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

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Or

The keyword or is a logical operator and used for combining conditions :

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

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Nested If

if statements are inside, this is called statements nested if declarations are inside.

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!

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The pass Statement

if statements cannot be empty but if you have an if statement without content for any reason, put the pass statement in order to avoid error.

Example :-

a = 33
b = 200

if b > a:
  pass

Note:- having an empty if statement like this, would raise an error without the pass statement.

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