If Statements and Chained Conditionals in Python 3
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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.
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.
Falserepresent the two Boolean truth values.
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
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 (boolean expression) then clause else clause end if
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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.
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:
ifkeyword begins the conditional statement.
- A conditional expression, which evaluates to a Boolean value of
False. The expression can optionally be enclosed in brackets.
:symbol marks the end of the line and the end of the
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.
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
not. It can also be the return value from a function. Any expression that evaluates to a Boolean value of
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
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
- File: if_temp1.py
1 2 3 4 5 6
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")
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
1 2 3 4 5
temperature = 55 print("The temperature is: ", temperature) if temperature > 65: print("This is a nice day.") print("End of program")
The temperature is: 55 End of program
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
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
not(officer == "Detective") is therefore
True. Python runs the code block and prints the line
the detective is not here.
- File: ifnot1.py
1 2 3 4
officer = "Constable" if not(officer == "Detective"): print("The detective is not here.") print("End of program")
The detective is not here. End of program
In this case,
officer is set to
officer == "Detective" is
not(officer == "Detective") is
False. Python does not run the code block, and
End of program is printed.
- File: ifnot2.py
1 2 3 4
officer = "Detective" if not(officer == "Detective"): print("The detective is not here.") print("End of program")
End of program
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
1 2 3 4 5
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")
This is a nice day and the detective is not here. End of program
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.
else directive is not a conditional statement. The decision about what code block to run occurs in the
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
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
1 2 3 4 5 6 7
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")
The temperature is: 55 The weather is too cold End of program
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.
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
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
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
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
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
1 2 3 4 5 6 7 8 9
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")
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
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
1 2 3 4 5 6 7 8 9
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")
The temperature is: 40 The weather is too cold End of program
matchstatement. This control structure compares an expression to a list of patterns using
matchstatement might be more suitable than the
elifstatement under some circumstances. See the Python Control Flow Documentation for more details.
“Nested if” statements are related but different. In a “nested if” statement, any of the
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.
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.
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