Does Python have a ternary conditional operator?

Yes, it was added in version 2.5. The expression syntax is:

a if condition else b

First condition is evaluated, then exactly one of either a or b is evaluated and returned based on the Boolean value of condition. If condition evaluates to True, then a is evaluated and returned but b is ignored, or else when b is evaluated and returned but a is ignored.

This allows short-circuiting because when condition is true only a is evaluated and b is not evaluated at all, but when condition is false only b is evaluated and a is not evaluated at all.

For example:

>>> 'true' if True else 'false'
'true'
>>> 'true' if False else 'false'
'false'

Note that conditionals are an expression, not a statement. This means you can't use statements such as pass, or assignments with = (or "augmented" assignments like +=), within a conditional expression:

>>> pass if False else pass
  File "<stdin>", line 1
    pass if False else pass
         ^
SyntaxError: invalid syntax

>>> # Python parses this as `x = (1 if False else y) = 2`
>>> # The `(1 if False else x)` part is actually valid, but
>>> # it can't be on the left-hand side of `=`.
>>> x = 1 if False else y = 2
  File "<stdin>", line 1
SyntaxError: cannot assign to conditional expression

>>> # If we parenthesize it instead...
>>> (x = 1) if False else (y = 2)
  File "<stdin>", line 1
    (x = 1) if False else (y = 2)
       ^
SyntaxError: invalid syntax

(In 3.8 and above, the := "walrus" operator allows simple assignment of values as an expression, which is then compatible with this syntax. But please don't write code like that; it will quickly become very difficult to understand.)

Similarly, because it is an expression, the else part is mandatory:

# Invalid syntax: we didn't specify what the value should be if the 
# condition isn't met. It doesn't matter if we can verify that
# ahead of time.
a if True

You can, however, use conditional expressions to assign a variable like so:

x = a if True else b

Or for example to return a value:

# Of course we should just use the standard library `max`;
# this is just for demonstration purposes.
def my_max(a, b):
    return a if a > b else b

Think of the conditional expression as switching between two values. We can use it when we are in a 'one value or another' situation, where we will do the same thing with the result, regardless of whether the condition is met. We use the expression to compute the value, and then do something with it. If you need to do something different depending on the condition, then use a normal if statement instead.

Keep in mind that it's frowned upon by some Pythonistas for several reasons:

• The order of the arguments is different from those of the classic condition ? a : b ternary operator from many other languages (such as C, C++, Go, Perl, Ruby, Java, JavaScript, etc.), which may lead to bugs when people unfamiliar with Python's "surprising" behaviour use it (they may reverse the argument order).
• Some find it "unwieldy", since it goes contrary to the normal flow of thought (thinking of the condition first and then the effects).
• Stylistic reasons. (Although the 'inline if' can be really useful, and make your script more concise, it really does complicate your code)

If you're having trouble remembering the order, then remember that when read aloud, you (almost) say what you mean. For example, x = 4 if b > 8 else 9 is read aloud as x will be 4 if b is greater than 8 otherwise 9.

Official documentation:

• Conditional expressions
• Is there an equivalent of C’s ”?:” ternary operator?

You can index into a tuple:

(falseValue, trueValue)[test]

test needs to return True or False.
It might be safer to always implement it as:

(falseValue, trueValue)[test == True]

or you can use the built-in bool() to assure a Boolean value:

(falseValue, trueValue)[bool(<expression>)]

For versions prior to 2.5, there's the trick:

[expression] and [on_true] or [on_false]

It can give wrong results when on_true has a false Boolean value.¹

Although it does have the benefit of evaluating expressions left to right, which is clearer in my opinion.

_{1. Is there an equivalent of C’s ”?:” ternary operator?}

From the documentation:

Conditional expressions (sometimes called a “ternary operator”) have the lowest priority of all Python operations.

The expression x if C else y first evaluates the condition, C (not x); if C is true, x is evaluated and its value is returned; otherwise, y is evaluated and its value is returned.

See PEP 308 for more details about conditional expressions.

New since version 2.5.

One of the alternatives to Python's conditional expression

"yes" if boolean else "no"

is the following:

{True: "yes", False: "no"}[boolean]

which has the following nice extension:

{True: "yes", False: "no", None: "maybe"}[boolean_or_none]

The shortest alternative remains

("no", "yes")[boolean]

which works because issubclass(bool, int).

Careful, though: the alternative to

yes() if boolean else no()

is not

(no(), yes())[boolean]  # bad: BOTH no() and yes() are called

but

(no, yes)[boolean]()

This works fine as long as no and yes are to be called with exactly the same parameters. If they are not, like in

yes("ok") if boolean else no()  # (1)

or in

yes("ok") if boolean else no("sorry")  # (2)

then a similar alternative either does not exist (1) or is hardly viable (2). (In rare cases, depending on the context, something like

msg = ("sorry", "ok")[boolean]
(no, yes)[boolean](msg)

could make sense.)

_{Thanks to Radek Rojík for his comment}

As already answered, yes, there is a ternary operator in Python:

<expression 1> if <condition> else <expression 2>

In many cases <expression 1> is also used as Boolean evaluated <condition>. Then you can use short-circuit evaluation.

a = 0
b = 1

# Instead of this:
x = a if a else b
# Evaluates as 'a if bool(a) else b'

# You could use short-circuit evaluation:
x = a or b

One big pro of short-circuit evaluation is the possibility of chaining more than two expressions:

x = a or b or c or d or e

When working with functions it is more different in detail:

# Evaluating functions:
def foo(x):
    print('foo executed')
    return x


def bar(y):
    print('bar executed')
    return y


def blubb(z):
    print('blubb executed')
    return z


# Ternary Operator expression 1 equals to False
print(foo(0) if foo(0) else bar(1))
''' foo and bar are executed once
foo executed
bar executed
1
'''

# Ternary Operator expression 1 equals to True
print(foo(2) if foo(2) else bar(3))
''' foo is executed twice!
foo executed
foo executed
2
'''

# Short-circuit evaluation second equals to True
print(foo(0) or bar(1) or blubb(2))
''' blubb is not executed
foo executed
bar executed
1
'''

# Short-circuit evaluation third equals to True
print(foo(0) or bar(0) or blubb(2))
'''
foo executed
bar executed
blubb executed
2
'''

# Short-circuit evaluation all equal to False
print(foo(0) or bar(0) or blubb(0))
''' Result is 0 (from blubb(0)) because no value equals to True
foo executed
bar executed
blubb executed
0
'''

PS: Of course, a short-circuit evaluation is not a ternary operator, but often the ternary is used in cases where the short circuit would be enough. It has a better readability and can be chained.

Simulating the Python ternary operator.

For example

a, b, x, y = 1, 2, 'a greather than b', 'b greater than a'
result = (lambda:y, lambda:x)[a > b]()

Output:

'b greater than a'

a if condition else b

Just memorize this pyramid if you have trouble remembering:

     condition
  if           else
a                   b 

Vinko Vrsalovic's answer is good enough. There is only one more thing:

Note that conditionals are an expression, not a statement. This means you can't use assignment statements or pass or other statements within a conditional expression

Walrus operator in Python 3.8

After the walrus operator was introduced in Python 3.8, something changed.

(a := 3) if True else (b := 5)

gives a = 3 and b is not defined,

(a := 3) if False else (b := 5)

gives a is not defined and b = 5, and

c = (a := 3) if False else (b := 5)

gives c = 5, a is not defined and b = 5.

Even if this may be ugly, assignments can be done inside conditional expressions after Python 3.8. Anyway, it is still better to use normal if statement instead in this case.

Many programming languages derived from C usually have the following syntax of the ternary conditional operator:

<condition> ? <expression1> : <expression2>

At first, the Python's benevolent dictator for life (I mean Guido van Rossum, of course) rejected it (as non-Pythonic style), since it's quite hard to understand for people not used to C language. Also, the colon sign : already has many uses in Python. After PEP 308 was approved, Python finally received its own shortcut conditional expression (what we use now):

<expression1> if <condition> else <expression2>

So, firstly it evaluates the condition. If it returns True, expression1 will be evaluated to give the result, otherwise expression2 will be evaluated. Due to lazy evaluation mechanics – only one expression will be executed.

Here are some examples (conditions will be evaluated from left to right):

pressure = 10
print('High' if pressure < 20 else 'Critical')

# Result is 'High'

Ternary operators can be chained in series:

pressure = 5
print('Normal' if pressure < 10 else 'High' if pressure < 20 else 'Critical')

# Result is 'Normal'

The following one is the same as previous one:

pressure = 5

if pressure < 20:
    if pressure < 10:
        print('Normal')
    else:
        print('High')
else:
    print('Critical')

# Result is 'Normal'

Yes, Python have a ternary operator, here is the syntax and an example code to demonstrate the same :)

#[On true] if [expression] else[On false]
# if the expression evaluates to true then it will pass On true otherwise On false

a = input("Enter the First Number ")
b = input("Enter the Second Number ")

print("A is Bigger") if a>b else print("B is Bigger")

Other answers correctly talk about the Python ternary operator. I would like to complement by mentioning a scenario for which the ternary operator is often used, but for which there is a better idiom. This is the scenario of using a default value.

Suppose we want to use option_value with a default value if it is not set:

run_algorithm(option_value if option_value is not None else 10)

or, if option_value is never set to a falsy value (0, "", etc.), simply

run_algorithm(option_value if option_value else 10)

However, in this case an ever better solution is simply to write

run_algorithm(option_value or 10)

The syntax for the ternary operator in Python is:

[on_true] if [expression] else [on_false]

Using that syntax, here is how we would rewrite the code above using Python’s ternary operator:

game_type = 'home'
shirt = 'white' if game_type == 'home' else 'green'

It's still pretty clear, but much shorter. Note that the expression could be any type of expression, including a function call, that returns a value that evaluates to True or False.

Python has a ternary form for assignments; however there may be even a shorter form that people should be aware of.

It's very common to need to assign to a variable one value or another depending on a condition.

>>> li1 = None
>>> li2 = [1, 2, 3]
>>>
>>> if li1:
...     a = li1
... else:
...     a = li2
...
>>> a
[1, 2, 3]

^ This is the long form for doing such assignments.

Below is the ternary form. But this isn't the most succinct way - see the last example.

>>> a = li1 if li1 else li2
>>>
>>> a
[1, 2, 3]
>>>

With Python, you can simply use or for alternative assignments.

>>> a = li1 or li2
>>>
>>> a
[1, 2, 3]
>>>

The above works since li1 is None and the interpreter treats that as False in logic expressions. The interpreter then moves on and evaluates the second expression, which is not None and it's not an empty list - so it gets assigned to a.

This also works with empty lists. For instance, if you want to assign a whichever list has items.

>>> li1 = []
>>> li2 = [1, 2, 3]
>>>
>>> a = li1 or li2
>>>
>>> a
[1, 2, 3]
>>>

Knowing this, you can simply such assignments whenever you encounter them. This also works with strings and other iterables. You could assign a whichever string isn't empty.

>>> s1 = ''
>>> s2 = 'hello world'
>>>
>>> a = s1 or s2
>>>
>>> a
'hello world'
>>>

I always liked the C ternary syntax, but Python takes it a step further!

I understand that some may say this isn't a good stylistic choice, because it relies on mechanics that aren't immediately apparent to all developers. I personally disagree with that viewpoint. Python is a syntax-rich language with lots of idiomatic tricks that aren't immediately apparent to the dabbler. But the more you learn and understand the mechanics of the underlying system, the more you appreciate it.

Pythonic way of doing the things:

"true" if var else "false"

But there always exists a different way of doing a ternary condition too:

"true" and var or "false"

The if else-if version can be written as:

sample_set="train" if "Train" in full_path else ("test" if "Test" in full_path else "validation")

There are multiple ways. The simplest one is to use the condition inside the "print" method.

You can use

print("Twenty" if number == 20 else "Not twenty")

Which is equivalent to:

if number == 20:
    print("Twenty")
else:
    print("Not twenty")

In this way, more than two statements are also possible to print. For example:

if number == 20:
    print("Twenty")
elif number < 20:
    print("Lesser")
elif 30 > number > 20:
    print("Between")
else:
    print("Greater")

can be written as:

print("Twenty" if number == 20 else "Lesser" if number < 20 else "Between" if 30 > number > 20 else "Greater")

Yes, it has, but it's different from C-syntax-like programming languages (which is condition ? value_if_true : value_if_false

In Python, it goes like this: value_if_true if condition else value_if_false

Example: even_or_odd = "even" if x % 2 == 0 else "odd"

A neat way to chain multiple operators:

f = lambda x,y: 'greater' if x > y else 'less' if y > x else 'equal'

array = [(0,0),(0,1),(1,0),(1,1)]

for a in array:
  x, y = a[0], a[1]
  print(f(x,y))

# Output is:
#   equal,
#   less,
#   greater,
#   equal

I find the default Python syntax val = a if cond else b cumbersome, so sometimes I do this:

iif = lambda (cond, a, b): a if cond else b
# So I can then use it like:
val = iif(cond, a, b)

Of course, it has the downside of always evaluating both sides (a and b), but the syntax is way clearer to me.