Introduction to theif Statement

We’ll start by looking at the most basic type ofif statement. In its simplest form, it looks like this:

if<expr>:<statement>

In the form shown above:

• <expr> is an expression evaluated in aBoolean context, as discussed in the section onLogical Operators in the Operators and Expressions in Python tutorial.
• <statement> is a valid Python statement, which must be indented. (You will see why very soon.)

If<expr> is true (evaluates to a value that is “truthy”), then<statement> is executed. If<expr> is false, then<statement> is skipped over and not executed.

Note that the colon (:) following<expr> is required. Some programming languages require<expr> to be enclosed in parentheses, but Python does not.

Here are several examples of this type ofif statement:

>>>x=0>>>y=5>>>ifx<y:# Truthy...print('yes')...yes>>>ify<x:# Falsy...print('yes')...>>>ifx:# Falsy...print('yes')...>>>ify:# Truthy...print('yes')...yes>>>ifxory:# Truthy...print('yes')...yes>>>ifxandy:# Falsy...print('yes')...>>>if'aul'in'grault':# Truthy...print('yes')...yes>>>if'quux'in['foo','bar','baz']:# Falsy...print('yes')...

Grouping Statements: Indentation and Blocks

So far, so good.

But let’s say you want to evaluate a condition and then do more than one thing if it is true:

If the weather is nice, then I will:

• Mow the lawn
• Weed the garden
• Take the dog for a walk

(If the weather isn’t nice, then I won’t do any of these things.)

In all the examples shown above, eachif <expr>: has been followed by only a single<statement>. There needs to be some way to say “If<expr> is true, do all of the following things.”

The usual approach taken by most programming languages is to define a syntactic device that groups multiple statements into onecompound statement orblock. A block is regarded syntactically as a single entity. When it is the target of anif statement, and<expr> is true, then all the statements in the block are executed. If<expr> is false, then none of them are.

Virtually all programming languages provide the capability to define blocks, but they don’t all provide it in the same way. Let’s see how Python does it.

Python: It’s All About the Indentation

Python follows a convention known as theoff-side rule, a term coined by British computer scientist Peter J. Landin. (The term is taken from the offside law in association football.) Languages that adhere to the off-side rule define blocks by indentation. Python is one of a relatively small set ofoff-side rule languages.

Recall from the previous tutorial on Python program structure thatindentation has special significance in a Python program. Now you know why: indentation is used to define compound statements or blocks. In a Python program, contiguous statements that are indented to the same level are considered to be part of the same block.

Thus, a compoundif statement in Python looks like this:

Python

 1if<expr>: 2<statement> 3<statement> 4... 5<statement> 6<following_statement>

Here, all the statements at the matching indentation level (lines 2 to 5) are considered part of the same block. The entire block is executed if<expr> is true, or skipped over if<expr> is false. Either way, execution proceeds with<following_statement> (line 6) afterward.

Notice that there is no token that denotes the end of the block. Rather, the end of the block is indicated by a line that is indented less than the lines of the block itself.

Note: In the Python documentation, a group of statements defined by indentation is often referred to as asuite. This tutorial series uses the terms block and suite interchangeably.

Consider this script filefoo.py:

Python

 1if'foo'in['bar','baz','qux']: 2print('Expression was true') 3print('Executing statement in suite') 4print('...') 5print('Done.') 6print('After conditional')

Runningfoo.py produces this output:

Windows Command Prompt

C:\> python foo.pyAfter conditional

The fourprint() statements on lines 2 to 5 are indented to the same level as one another. They constitute the block that would be executed if the condition were true. But it is false, so all the statements in the block are skipped. After the end of the compoundif statement has been reached (whether the statements in the block on lines 2 to 5 are executed or not), execution proceeds to the first statement having a lesser indentation level: theprint() statement on line 6.

Blocks can be nested to arbitrary depth. Each indent defines a new block, and each outdent ends the preceding block. The resulting structure is straightforward, consistent, and intuitive.

Here is a more complicated script file calledblocks.py:

Python

# Does line execute?                        Yes    No#                                           ---    --if'foo'in['foo','bar','baz']:#  xprint('Outer condition is true')#  xif10>20:#  xprint('Inner condition 1')#        xprint('Between inner conditions')#  xif10<20:#  xprint('Inner condition 2')#  xprint('End of outer condition')#  xprint('After outer condition')#  x

The output generated when this script is run is shown below:

Windows Command Prompt

C:\> python blocks.pyOuter condition is trueBetween inner conditionsInner condition 2End of outer conditionAfter outer condition

Note: In case you have been wondering, the off-side rule is the reason for the necessity of the extra newline when entering multiline statements in a REPL session. The interpreter otherwise has no way to know that the last statement of the block has been entered.

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What Do Other Languages Do?

Perhaps you’re curious what the alternatives are. How are blocks defined in languages that don’t adhere to the off-side rule?

The tactic used by most programming languages is to designate special tokens that mark the start and end of a block. For example, in Perl blocks are defined with pairs of curly braces ({}) like this:

Perl

# (This is Perl, not Python)if(<expr>){<statement>;<statement>;...<statement>;}<following_statement>;

C/C++,Java, and a whole host ofother languages use curly braces in this way.

Other languages, such as Algol and Pascal, use keywordsbegin andend to enclose blocks.

Theelse andelif Clauses

Now you know how to use anif statement to conditionally execute a single statement or a block of several statements. It’s time to find out what else you can do.

Sometimes, you want to evaluate a condition and take one path if it is true but specify an alternative path if it is not. This is accomplished with anelse clause:

if<expr>:<statement(s)>else:<statement(s)>

If<expr> is true, the first suite is executed, and the second is skipped. If<expr> is false, the first suite is skipped and the second is executed. Either way, execution then resumes after the second suite. Both suites are defined by indentation, as described above.

In this example,x is less than50, so the first suite (lines 4 to 5) are executed, and the second suite (lines 7 to 8) are skipped:

 1>>>x=20 2 3>>>ifx<50: 4...print('(first suite)') 5...print('x is small') 6...else: 7...print('(second suite)') 8...print('x is large') 9...10(first suite)11x is small

Here, on the other hand,x is greater than50, so the first suite is passed over, and the second suite executed:

 1>>>x=120 2>>> 3>>>ifx<50: 4...print('(first suite)') 5...print('x is small') 6...else: 7...print('(second suite)') 8...print('x is large') 9...10(second suite)11x is large

There is also syntax for branching execution based on several alternatives. For this, use one or moreelif (short forelse if) clauses. Python evaluates each<expr> in turn and executes the suite corresponding to the first that is true. If none of the expressions are true, and anelse clause is specified, then its suite is executed:

if<expr>:<statement(s)>elif<expr>:<statement(s)>elif<expr>:<statement(s)>...else:<statement(s)>

An arbitrary number ofelif clauses can be specified. Theelse clause is optional. If it is present, there can be only one, and it must be specified last:

>>>name='Joe'>>>ifname=='Fred':...print('Hello Fred')...elifname=='Xander':...print('Hello Xander')...elifname=='Joe':...print('Hello Joe')...elifname=='Arnold':...print('Hello Arnold')...else:...print("I don't know who you are!")...Hello Joe

At most, one of the code blocks specified will be executed. If anelse clause isn’t included, and all the conditions are false, then none of the blocks will be executed.

>>>names={...'Fred':'Hello Fred',...'Xander':'Hello Xander',...'Joe':'Hello Joe',...'Arnold':'Hello Arnold'...}>>>print(names.get('Joe',"I don't know who you are!"))Hello Joe>>>print(names.get('Rick',"I don't know who you are!"))I don't know who you are!

Anif statement withelif clauses uses short-circuit evaluation, analogous to what you saw with theand andor operators. Once one of the expressions is found to be true and its block is executed, none of the remaining expressions are tested. This is demonstrated below:

>>>var# Not definedTraceback (most recent call last):  File"<pyshell#58>", line1, in<module>varNameError:name 'var' is not defined>>>if'a'in'bar':...print('foo')...elif1/0:...print("This won't happen")...elifvar:...print("This won't either")...foo

The second expression contains a division by zero, and the third references an undefinedvariablevar. Either would raise an error, but neither is evaluated because the first condition specified is true.

One-Lineif Statements

It is customary to writeif <expr> on one line and<statement> indented on the following line like this:

if<expr>:<statement>

But it is permissible to write an entireif statement on one line. The following is functionally equivalent to the example above:

if<expr>:<statement>

There can even be more than one<statement> on the same line, separated by semicolons:

if<expr>:<statement_1>;<statement_2>;...;<statement_n>

But what does this mean? There are two possible interpretations:

1. If<expr> is true, execute<statement_1>.

   Then, execute<statement_2> ... <statement_n> unconditionally, irrespective of whether<expr> is true or not.

2. If<expr> is true, execute all of<statement_1> ... <statement_n>. Otherwise, don’t execute any of them.

Python takes the latter interpretation. The semicolon separating the<statements> has higher precedence than the colon following<expr>—in computer lingo, the semicolon is said to bind more tightly than the colon. Thus, the<statements> are treated as a suite, and either all of them are executed, or none of them are:

>>>if'f'in'foo':print('1');print('2');print('3')...123>>>if'z'in'foo':print('1');print('2');print('3')...

Multiple statements may be specified on the same line as anelif orelse clause as well:

>>>x=2>>>ifx==1:print('foo');print('bar');print('baz')...elifx==2:print('qux');print('quux')...else:print('corge');print('grault')...quxquux>>>x=3>>>ifx==1:print('foo');print('bar');print('baz')...elifx==2:print('qux');print('quux')...else:print('corge');print('grault')...corgegrault

While all of this works, and the interpreter allows it, it is generally discouraged on the grounds that it leads to poor readability, particularly for complexif statements.PEP 8 specifically recommends against it.

As usual, it is somewhat a matter of taste. Most people would find the following more visually appealing and easier to understand at first glance than the example above:

>>>x=3>>>ifx==1:...print('foo')...print('bar')...print('baz')...elifx==2:...print('qux')...print('quux')...else:...print('corge')...print('grault')...corgegrault

If anif statement is simple enough, though, putting it all on one line may be reasonable. Something like this probably wouldn’t raise anyone’s hackles too much:

debugging=True# Set to True to turn debugging on....ifdebugging:print('About to call function foo()')foo()

Conditional Expressions (Python’s Ternary Operator)

Python supports one additional decision-making entity called a conditional expression. (It is also referred to as a conditional operator or ternary operator in various places in the Python documentation.) Conditional expressions were proposed for addition to the language inPEP 308 and green-lighted by Guido in 2005.

In its simplest form, the syntax of the conditional expression is as follows:

<expr1>if<conditional_expr>else<expr2>

This is different from theif statement forms listed above because it is not a control structure that directs the flow of program execution. It acts more like an operator that defines an expression. In the above example,<conditional_expr> is evaluated first. If it is true, the expression evaluates to<expr1>. If it is false, the expression evaluates to<expr2>.

Notice the non-obvious order: the middle expression is evaluated first, and based on that result, one of the expressions on the ends is returned. Here are some examples that will hopefully help clarify:

>>>raining=False>>>print("Let's go to the",'beach'ifnotrainingelse'library')Let's go to the beach>>>raining=True>>>print("Let's go to the",'beach'ifnotrainingelse'library')Let's go to the library>>>age=12>>>s='minor'ifage<21else'adult'>>>s'minor'>>>'yes'if('qux'in['foo','bar','baz'])else'no''no'

A common use of the conditional expression is to select variable assignment. For example, suppose you want to find the larger of two numbers. Of course, there is a built-in function,max(), that does just this (and more) that you could use. But suppose you want to write your own code from scratch.

You could use a standardif statement with anelse clause:

>>>ifa>b:...m=a...else:...m=b...

But a conditional expression is shorter and arguably more readable as well:

>>>m=aifa>belseb

Remember that the conditional expression behaves like an expression syntactically. It can be used as part of a longer expression. The conditional expression has lower precedence than virtually all the other operators, so parentheses are needed to group it by itself.

In the following example, the+ operator binds more tightly than the conditional expression, so1 + x andy + 2 are evaluated first, followed by the conditional expression. The parentheses in the second case are unnecessary and do not change the result:

>>>x=y=40>>>z=1+xifx>yelsey+2>>>z42>>>z=(1+x)ifx>yelse(y+2)>>>z42

If you want the conditional expression to be evaluated first, you need to surround it with grouping parentheses. In the next example,(x if x > y else y) is evaluated first. The result isy, which is40, soz is assigned1 + 40 + 2 =43:

>>>x=y=40>>>z=1+(xifx>yelsey)+2>>>z43

If you are using a conditional expression as part of a larger expression, it probably is a good idea to use grouping parentheses for clarification even if they are not needed.

Conditional expressions also useshort-circuit evaluation like compound logical expressions. Portions of a conditional expression are not evaluated if they don’t need to be.

In the expression<expr1> if <conditional_expr> else <expr2>:

• If<conditional_expr> is true,<expr1> is returned and<expr2> is not evaluated.
• If<conditional_expr> is false,<expr2> is returned and<expr1> is not evaluated.

As before, you can verify this by using terms that would raise an error:

>>>'foo'ifTrueelse1/0'foo'>>>1/0ifFalseelse'bar''bar'

In both cases, the1/0 terms are not evaluated, so no exception is raised.

Conditional expressions can also be chained together, as a sort of alternativeif/elif/else structure, as shown here:

>>>s=('foo'if(x==1)else...'bar'if(x==2)else...'baz'if(x==3)else...'qux'if(x==4)else...'quux'...)>>>s'baz'

It’s not clear that this has any significant advantage over the correspondingif/elif/else statement, but it is syntactically correct Python.

The Pythonpass Statement

Occasionally, you may find that you want to write what is called a code stub: a placeholder for where you will eventually put a block of code that you haven’t implemented yet.

In languages where token delimiters are used to define blocks, like the curly braces in Perl and C, empty delimiters can be used to define a code stub. For example, the following is legitimate Perl or C code:

# This is not Pythonif(x){}

Here, the empty curly braces define an empty block. Perl or C will evaluate the expressionx, and then even if it is true, quietly do nothing.

Because Python uses indentation instead of delimiters, it is not possible to specify an empty block. If you introduce anif statement withif <expr>:, something has to come after it, either on the same line or indented on the following line.

Consider this scriptfoo.py:

ifTrue:print('foo')

If you try to runfoo.py, you’ll get this:

C:\> python foo.py  File "foo.py", line 3    print('foo')        ^IndentationError: expected an indented block

ThePythonpass statement solves this problem. It doesn’t change program behavior at all. It is used as a placeholder to keep the interpreter happy in any situation where a statement is syntactically required, but you don’t really want to do anything:

ifTrue:passprint('foo')

Nowfoo.py runs without error:

C:\> python foo.pyfoo

Conclusion

With the completion of this tutorial, you are beginning to write Python code that goes beyond simple sequential execution:

• You were introduced to the concept ofcontrol structures. These are compound statements that alter programcontrol flow—the order of execution of program statements.
• You learned how to group individual statements together into ablock orsuite.
• You encountered your first control structure, theif statement, which makes it possible toconditionally execute a statement or block based on evaluation of program data.

All of these concepts are crucial to developing more complex Python code.

The next two tutorials will present two new control structures: thewhile statement and thefor statement. These structures facilitateiteration, execution of a statement or block of statements repeatedly.

Take the Quiz: Test your knowledge with our interactive “Python Conditional Statements” quiz. You’ll receive a score upon completion to help you track your learning progress:

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Interactive Quiz

Python Conditional Statements

Test your understanding of Python conditional statements