More on conditionals in Python

Previously, you explored conditional statements and how they’re useful in automating tasks in Python. So far, you’ve focused on the if and else keywords. In this reading, you’ll review these and learn another keyword, elif. You’ll also learn how you can apply the and, or, and not operators to your conditions.

How conditional statements work

A conditional statement is a statement that evaluates code to determine whether it meets a specific set of conditions. When a condition is met, it evaluates to a Boolean value of True and performs specified actions. When the condition isn’t met, it evaluates a Boolean value of False and doesn’t perform the specified actions.

In conditional statements, the condition is often based on a comparison of two values. This table summarizes common comparison operators used to compare numerical values.

Note: The equal to (==) and not equal to (!=) operators are also commonly used to compare string data.

if statements

The keyword if starts a conditional statement. It’s a necessary component of any conditional statement. In the following example, if begins a statement that tells Python to print an "OK" message when the HTTP response status code equals 200:

if status == 200:

    print("OK")

This code consists of a header and a body.

The header of an if statement

The first line of this code is the header. In the header of an if statement, the keyword if is followed by the condition. Here, the condition is that the status variable is equal to a value of 200. The condition can be placed in parentheses:

if (status == 200):

    print("OK")

In cases like this one, placing parentheses around conditions in Python is optional. You might want to include them if it helps you with code readability. However, this condition will be processed the same way if written without parentheses. 

In other situations, because Python evaluates the conditions in parentheses first, parentheses can affect how Python processes conditions. You will read more about one of these in the section of this reading on not.

Note: You must always place a colon (:) at the end of the header. Without this syntax, the code will produce an error.

The body of an if statement

After the header of an if statement comes the body of the if statement. This tells Python what action or actions to perform when the condition evaluates to True. In this example, there is just one action, printing "OK" to the screen. In other cases, there might be more lines of code with additional actions.

Note: For the body of the if statement to execute as intended, it must be indented further than the header. Additionally, if there are multiple lines of code within the body, they must all be indented consistently. 

Continuing conditionals with else and elif

In the previous example, if the HTTP status response code was not equal to 200, the condition would evaluate to False and Python would continue with the rest of the program. However, it’s also possible to specify alternative actions with else and elif.

else statements

The keyword else precedes a code section that only evaluates when all conditions that precede it within the conditional statement evaluate to False.

In the following example, when the HTTP response status code is not equal to 200, it prints an alternative message of "check other status":

if status == 200:

    print("OK")

else:

    print("check other status")

Note: Like with if, a colon (:) is required after else, and the body that follows the else header is indented.

elif statements

In some cases, you might have multiple alternative actions that depend on new conditions. In that case, you can use elif. The elif keyword precedes a condition that is only evaluated when previous conditions evaluate to False. Unlike with else, there can be multiple elif statements following if.

For example, you might want to print one message if the HTTP response status code is 200, one message if it is 400, and one if it is 500. The following code demonstrates how you can use elif for this: 

if status == 200:

    print("OK")

elif status == 400:

    print("Bad Request")

elif status == 500:

    print("Internal Server Error") 

Python will first check if the value of status is 200, and if this evaluates to False, it will go onto the first elif statement. There, it will check whether the value of status is 400. If that evaluates to True, it will print "Bad Request", but if it evaluates to False, it will go on to the next elif statement. 

If you want the code to print another message when all conditions evaluate to False, then you can incorporate else after the last elif. In this example, if it reaches the else statement, it prints a message to check the status:

if status == 200:

    print("OK")

elif status == 400:

    print("Bad Request")

elif status == 500:

    print("Internal Server Error")

else:

    print("check other status")

Just like with if and else, it’s important to place a colon (:) after the elif header and indent the code that follows this header.

Note: Python processes multiple elif statements differently than multiple if statements. When it reaches an elif statement that evaluates to True, it won’t check the following elif statements. On the other hand, Python will run all if statements.

Logical operators for multiple conditions

In some cases, you might want Python to perform an action based on a more complex condition. You might require two conditions to evaluate to True. Or, you might require only one of two conditions to evaluate to True. Or, you might want Python to perform an action when a condition evaluates to False. The operators and, or, and not can be used in these cases.

and

The and operator requires both conditions on either side of the operator to evaluate to True. For example, all HTTP status response codes between 200 and 226 relate to successful responses. You can use and to join a condition of being greater than or equal to 200 with another condition of being less than or equal to 226:

if status >= 200 and status <= 226:

    print("successful response")

When both conditions are True, then the "successful response" message will print.

or

The or operator requires only one of the conditions on either side of the operator to evaluate to True. For example, both a status code of 100 and a status code of 102 are informational responses. Using or, you could ask Python to print an "informational response" message when the code is either 100 or 102:

if status == 100 or status == 102:

    print("informational response")

Only one of these conditions needs to be met for Python to print the message.

not

The not operator negates a given condition so that it evaluates to False if the condition is True and to True if it is False. For example, if you want to indicate that Python should check the status code when it’s something outside of the successful range, you can use not:

if not(status >= 200 and status <= 226):

    print("check status")

Python first checks whether the value of status is greater than or equal to 200 and less than or equal to 226, and then because of the operator not, it inverts this. This means it will print the message if status is less than 200 or greater than 226.

Note: In this case, the parentheses are necessary for the code to apply not to both conditions. Python will evaluate the conditions within the parentheses first. This means it will first evaluate the conditions on either side of the and operator and then apply not to both of them.

Key takeaways

It’s important for security analysts to be familiar with conditional statements. Conditional statements require the if keyword. You can also use else and elif when working with conditionals to specify additional actions to take. The logical operators and, or, and not are also useful when writing conditionals.