Conditionals ¶

Python has all of the standard comparison operators shown below.

< is less than.

In [1]:

print(3 < 5)
print(-1 < 0)
print(5 < 3)
print(3 < 3)

print(3 < 5) print(-1 < 0) print(5 < 3) print(3 < 3)

True
True
False
False

<= is less than or equal to.

In [2]:

print(4 <= 5)
print(5 <= 5)
print(6 <= 5)

print(4 <= 5) print(5 <= 5) print(6 <= 5)

True
True
False

> is greater than.

In [3]:

print(4 > 5)
print(5 > 5)
print(6 > 5)

print(4 > 5) print(5 > 5) print(6 > 5)

False
False
True

>= is great than or equal to.

In [4]:

print(4 >= 5)
print(5 >= 5)
print(6 >= 5)

print(4 >= 5) print(5 >= 5) print(6 >= 5)

False
True
True

== is equal to.

In [5]:

print(4 == 5)
print(5 == 5)
print(5 == 5.0)
print(5 == "5")

print(4 == 5) print(5 == 5) print(5 == 5.0) print(5 == "5")

False
True
True
False

Note that I started mixing up the data types. For example, 5 == 5.0 is True but 5 == "5" is not. this again has to do with comparing different data types. Python can reason that an int can be turned into a float for comparison, but it is not going to even try to convert a string to a numeric type. However, instead of throwing an error it just returns False .

!= is not equal to.

• 4 != 5 is True
• 5 != 5 is False
• "5" != 5 is True

In [6]:

print(4 != 5)
print(5 != 5)
print("5" != 5)

print(4 != 5) print(5 != 5) print("5" != 5)

True
False
True

However, none of this means I cannot compare string.

In [7]:

print("Hello" == "Hello")
print("Hello " == "Hello")
print("Whats up" != "doc")

print("Hello" == "Hello") print("Hello " == "Hello") print("Whats up" != "doc")

True
False
True

if ¶

In programming, an essential construct for controlling the flow of your code is the if statement, more formally known as a conditional statement. This powerful tool allows you to control whether a specific block of code is used based on something is True or False , called the condition. We hinted at this in functions where we said 'if the tool is equal to "whisk" , then grab a whisk'. In Python, this would look like this.

if tool == "whisk":
    grab_whisk()

The == operator compares two things and sees if it is equal; this is different than assigning a variable with = . In our case, if the data stored in the variable tool is "whisk" , then tool == "whisk" would evaluate to True . Since this is True , we go "inside the if statement" specified after the : and indented by four spaces.

In [8]:

number = 10

if number > 0:
    print("The number is positive.")

print("Done!")

number = 10 if number > 0: print("The number is positive.") print("Done!")

The number is positive.
Done!

In the case above, Python will compare 10 > 0 and see that it is True and go inside the if statement. Remember that we have to tell Python when we have stopped defining code after : ? Well, we do it there as well where print("Done!") is not indented and will run no matter is number > 0 .

In [9]:

number = -10

if number > 0:
    print("The number is positive.")

print("Done!")

number = -10 if number > 0: print("The number is positive.") print("Done!")

Done!

else ¶

In Python, the else statement serves as a dynamic tool to extend the functionality of if statements, providing a means to execute a block of code when the specified condition in the if statement evaluates to False . This invaluable feature enhances the flexibility and control you have over your code execution.

The else statement is typically used in conjunction with an if statement. It follows the body of the if block and introduces an alternative block of code to be executed when the condition of the if statement is not met.

In [10]:

number = -10

if number > 0:
    print("The number is positive.")
else:
    print("The number is not positive.")

print("Done!")

number = -10 if number > 0: print("The number is positive.") else: print("The number is not positive.") print("Done!")

The number is not positive.
Done!

In [11]:

number = 10

if number > 0:
    print("The number is positive.")
else:
    print("The number is not positive.")

print("Done!")

number = 10 if number > 0: print("The number is positive.") else: print("The number is not positive.") print("Done!")

The number is positive.
Done!

elif ¶

In Python, the elif (short for "else if") statement emerges as a powerful tool for introducing multiple tests within the decision-making process. This feature is especially useful when you need to provide several choices, each contingent on its own set of conditions. Understanding the nuances of elif is key to enhancing the flexibility and sophistication of your programs.

For example, what about 0? Zero is not positive, or negative, so we have another conditional.

In [12]:

number = 0

if number > 0:
    print("The number is positive.")
elif number == 0:
    print("The number is zero.")
else:
    print("The number is negative.")

print("Done!")

number = 0 if number > 0: print("The number is positive.") elif number == 0: print("The number is zero.") else: print("The number is negative.") print("Done!")

The number is zero.
Done!

Note that we can change our conditionals if we like. We will still get the same behavior.

In [13]:

number = 0

if number > 0:
    print("The number is positive.")
elif number < 0:
    print("The number is negative.")
else:
    print("The number is zero.")

print("Done!")

number = 0 if number > 0: print("The number is positive.") elif number < 0: print("The number is negative.") else: print("The number is zero.") print("Done!")

The number is zero.
Done!