python conditions

Python Conditions 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")



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Example

If statement:

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):

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")



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Example

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")



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Example

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")



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Example

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:



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



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Example

One line if statement:

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

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Example

One line if else statement:

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

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Example

One line if else statement, with 3 conditions:

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 = 200b = 33c = 500if a > b and c > a:

 

print("Both conditions are True")



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Example

Test if a is greater than

b, AND if c

is greater than a:

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 = 200b = 33c = 500if a > b or a > c:

 

print("At least one of the conditions is True")



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Example

Test if a is greater than

b, OR if a

is greater than c:

Nested If

You can have if statements inside

if statements, this is called nested

if statements.

Example



x = 41if x > 10:

 

print("Above ten,")  if x > 20:    print("and

also above 20!")  else:    print("but not

above 20.")

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Example

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 = 33b = 200if b > a:  pass

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Example

Test Yourself With Exercises

Exercise:

Print "Hello World if a is greater than b.

Start the Exercise