If Statements and Chained Conditionals in Python 3

Python programs must be able to run different branches of code in different situations. This is usually accomplished through the use of conditional statements, which determine the control flow through a program. Python’s if statement is used to decide whether or not some code should run. This guide explains the if statement and other Python conditionals and demonstrates how to use them.

An Introduction to Conditional Statements

Conditional statements are programming structures that can make decisions. Without conditionals and other control statements, a program would execute in a deterministic manner, one statement after the next, every time. Conditionals allow different inputs to directly affect the program’s behavior. They allow programmers to construct more sophisticated, powerful, and useful programs, and are essential to computer science.

People make conditional decisions every day. If it is raining, they take an umbrella. If it is a workday, they get up early. Otherwise, they sleep in. Conditionals work the same way in computing. A conditional statement evaluates a Boolean expression and calculates whether it is true or false. This result affects the flow of the program. If the expression is true, the program runs a certain block of code. If necessary, it can execute a different block when the conditional is false.

A conditional statement typically follows an if then format. The if component is paired with a conditional expression and a block of code. If the conditional is true, the program runs the code inside the block. This code block is sometimes referred to as the clause. If the conditional is false, the block is not executed. Sometimes the conditional has an if-then-else format. The else branch contains a code block that only runs when the conditional is false.

A conditional statement can be used whenever different actions should be taken based on different input conditions. The list below includes some programming scenarios that lend themselves to conditional statements.

• An item should be displayed only if it falls within a specified price range.
• If a customer has been validated, then send them to a payment screen. Otherwise, ask for their credentials.
• A user is prompted to delete any documents that have not been updated within the last year.

In pseudocode, the structure of an if then conditional follows the pattern below:

if (boolean expression) then
    clause
end if

An if then conditional can be extended using the else option to form an if then else statement. There is no conditional expression associated with the else component. The program makes all decisions when it evaluates the if expression.

if (boolean expression) then
    clause
else
    clause
end if

Before You Begin

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Python Conditionals

To implement conditionals in Python, use the if statement. The Python if statement takes a variety of forms. It can be combined with an elif statement, which stands for “else if”, or with an else option. The following sections explain how to best use the Python if statement in different circumstances.

The Python If Statement

The Python if statement runs a block of code if and only if certain conditions are met. It is structured as a compound statement. This means it contains a header and an associated suite. The first line of the if statement is the header. The if header consists of the following three components, in the following order:

1. The if keyword begins the conditional statement.
2. A conditional expression, which evaluates to a Boolean value of True or False. The expression can optionally be enclosed in brackets.
3. The : symbol marks the end of the line and the end of the if statement header.

The suite follows the header. It contains one or more lines of code to execute and must be indented. This indented section is also known as the code block or the conditional clause. There is no limit to the length of this block, which is terminated by the next non-indented line. According to Python’s PEP 8 style guidelines, four spaces should be used for the indentation.

The if statement evaluates the conditional. If the conditional is True, it runs the corresponding code block. If the conditional expression is False, it does not do anything. The indented block is not executed, and the control flow moves to the next non-indented line.

The conditional expression can be quite complex. Comparison operators including the equality == operator and the “greater than” > operator are commonly used. But the expression can include logical boolean operators like and, or, and not. It can also be the return value from a function. Any expression that evaluates to a Boolean value of True or False is permitted.

A simple Python if statement looks like this:

if boolean_expression:
    command

When the Python interpreter encounters the if keyword, it evaluates the boolean_expression. If the result is True, Python runs the command.

Python if Example

The examples in this section demonstrate how to use the Python if command. The value of temperature is initially set to 75. Inside the if statement, Python analyzes the Boolean expression, temperature > 65 and decides it is True. Because the expression is True, Python executes the code block, which consists of two print statements. If the code block required more instructions, they would also be indented.

File: if_temp1.py

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temperature = 75 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") print("You should go outside.") print("End of program")

python3 if_temp1.py

The temperature is:  75
This is a nice day.
You should go outside.
End of program

The second example illustrates what happens when the conditional is False. The value of temperature is now only 55, so the conditional expression evaluates to False. The conditional statement is not satisfied, the code block is not executed, and the statement about the nice day is not printed. The control flow passes directly to the final line of the program, which prints End of program.

File: if_temp2.py

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temperature = 55 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") print("End of program")

python3 if_temp2.py

The temperature is:  55
End of program

Python “if not” Example

There are occasions where a block of code should only run if a condition is not met. To accomplish this, precede the conditional expression with the not keyword and enclose the expression in brackets.

Python evaluates the entire expression, including the not operator, to determine the truth value. It first analyzes the expression inside the brackets. It then feeds the result to the not operator. This operator calculates a result of either True or False, which is the final result of the conditional expression. The subsequent code block is only executed if the conditional is satisfied.

In this example, the if not conditional is only true when officer is not set to Detective. The comparison officer == "Detective" is False. not(officer == "Detective") is therefore True. Python runs the code block and prints the line the detective is not here.

File: ifnot1.py

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officer = "Constable" if not(officer == "Detective"): print("The detective is not here.") print("End of program")

python3 ifnot1.py

The detective is not here.
End of program

In this case, officer is set to detective. officer == "Detective" is True and not(officer == "Detective") is False. Python does not run the code block, and End of program is printed.

File: ifnot2.py

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officer = "Detective" if not(officer == "Detective"): print("The detective is not here.") print("End of program")

python3 ifnot2.py

End of program

Python “and” Example

A conditional can be built up into a complex compound expression involving several operators and clauses. This allows for more complex decisions to be made. The if code block is only executed if two, three, or even more conditions are met.

In this example, the code block associated with the if statement is only executed if two conditions are both true. The program uses a logical and expression to verify both expressions are True. Brackets are used to pre-calculate both inputs for the and operator. The line This is a nice day and the detective is not here is only printed when both prerequisites are satisfied. If either condition is False, the line is not printed.

File: if_and.py

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temperature = 75 officer = "Constable" if (temperature > 65) and (not(officer == "Detective")): print("This is a nice day and the detective is not here.") print("End of program")

python3 if_and.py

This is a nice day and the detective is not here.
End of program

Python If Else Statement

The Python if else statement allows a program to choose between one of two code paths. It adds an else code block that only runs when the conditional is False. In an if else statement, either the if code block or the else code block are executed, but not both. It is not possible to have a case where neither block is run. Each section must have at least one statement in the indentation block.

Technically, the else directive is not a conditional statement. The decision about what code block to run occurs in the if statement.

An if else statement in Python is structured like this:

if boolean_expression:
    command_1
else:
    command_2

The Python interpreter evaluates the boolean_expression associated with the if statement. If it is True, it runs command_1 but does not run command_2. Otherwise, it skips directly to the else code block and executes command_2.

The following example modifies the original if_temp2.py file to add an else clause. If the value of temperature satisfies the conditional clause, the if code block is executed. However, if temperature falls too low, the conditional statement becomes False. In this case, an alternative statement about colder weather is printed. In this example, temperature is only 55. The conditional statement temperature > 65 is not satisfied, so the control flow falls through to the else block. Python prints the line The weather is too cold".

File: ifelse_temp.py

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temperature = 55 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") else: print("The weather is too cold") print("End of program")

python3 ifelse_temp.py

The temperature is:  55
The weather is too cold
End of program

Python Chained Conditionals Using elif

The Python elif statement stands for “else if”. It is used to evaluate multiple expressions and choose from one of several different code paths. It is always used in conjunction with the if statement, and is sometimes referred to as a chained conditional.

Python first evaluates the if conditional. If the if conditional is False, Python inspects each elif conditional in sequential order until one of them evaluates to True. It then runs the corresponding elif code block. If all the elif conditionals are False, Python does not run any of the elif code blocks.

A sequence of elif statements can be followed by an optional else directive, which terminates the chain. The else code block is only executed when the if conditional and all elif conditionals are False. There is no limit to the number of elif expressions that can be used, but only one code block can ever be executed.

The Python if elif statement follows this template. The final else directive and code block are optional.

if boolean_expression:
    command_1
elif boolean_expression2:
    command_2
elif boolean_expression3:
    command_3
else:
    command_4

The ifelse_temp.py file from the previous section can be modified to include an elif statement. This allows for more effective processing of middling temperatures which are neither warm nor cold. A temperature value of more than 65 satisfies the if conditional and is still “nice”. However, a temperature of between 50 and 64 is now considered “okay”. A value in this range fails the if conditional but passes the elif conditional. The “cold” designation is reserved for temperatures of 50 or below. “Cold” temperatures below 50 evaluate to False in both the if and elif conditionals. Processing falls through to the else code block. Adding more conditions allows the program to more accurately represent the data.

In the first example, a temperature of 55 fails the first conditional test because it is less than 65. However, being greater than 50, it satisfies the elif conditional. Therefore, the line This is an okay day is printed.

File: ifelif_temp.py

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temperature = 55 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") elif temperature > 50: print("This is an okay day.") else: print("The weather is too cold") print("End of program")

python3 ifelif_temp.py

The temperature is:  55
This is an okay day.
End of program

In a follow-up run, the temperature is set to 40. Now both the if and elif conditionals evaluate to False. The control flow falls through to the else code block, where The weather is too cold is printed.

File: ifelif2_temp.py

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temperature = 40 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") elif temperature > 50: print("This is an okay day.") else: print("The weather is too cold") print("End of program")

python3 ifelif2_temp.py

The temperature is:  40
The weather is too cold
End of program

“Nested if” statements are related but different. In a “nested if” statement, any of the if, elif, or else code blocks can also contain an if statement. Although this structure is more complicated to write and understand, the same principles apply. Try to avoid using deeply nested control structures because the code can become difficult to read and debug. If there are more than two nested if statements, consider rewriting the code using functions or compound conditionals.

A Summary of Python Conditional Statements

The Python conditional statements play a central role in how the programming language is used. They allow a program to follow different paths under different conditions. Boolean expressions use Python’s logical and comparison operators to calculate a truth value and make a decision regarding what code block to execute.

Python implements conditionals using the if statement. A Python if statement first determines whether its conditional expression is True or not. If the result is True, it runs the corresponding code block. If the result is False, it does nothing. The Python if else statement still executes the if code block if the conditional is True, but it runs the else code block when the conditional is False. One of the more optional elif statements, signifying “else if”, can follow the if statement to allow for different code paths based on multiple comparisons. For more information on the Python control structures, see the official Python documentation.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Python control flow documentation
• PEP 8 Style Guidelines