Python 有三元条件运算符吗？

python
#1

如果 Python 没有三元条件运算符，是否可以使用其他语言构造来模拟一个？

#2

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>)]

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#3

Ternary Operator in different programming Languages

Here I just try to show some important difference in ternary operator between a couple of programming languages.

Ternary Operator in Javascript

var a = true ? 1 : 0;
# 1
var b = false ? 1 : 0;
# 0

Ternary Operator in Ruby

a = true ? 1 : 0
# 1
b = false ? 1 : 0
# 0

Ternary operator in Scala

val a = true ? 1 | 0
# 1
val b = false ? 1 | 0
# 0

Ternary operator in R programming

a <- if (TRUE) 1 else 0
# 1
b <- if (FALSE) 1 else 0
# 0

Ternary operator in Python

a = 1 if True else 0
# 1
b = 1 if False else 0
# 0

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#4

You might often find

cond and on_true or on_false

but this lead to problem when on_true == 0

>>> x = 0
>>> print x == 0 and 0 or 1 
1
>>> x = 1
>>> print x == 0 and 0 or 1 
1

where you would expect for a normal ternary operator this result

>>> x = 0
>>> print 0 if x == 0 else 1 
0
>>> x = 1
>>> print 0 if x == 0 else 1 
1

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#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]

but there is no alternative to

yes() if boolean else no()

if you want to avoid the evaluation of yes() and no(), because in

(no(), yes())[boolean]  # bad

both no() and yes() are evaluated.

#6 
a if condition else b

Just memorize this pyramid if you have trouble remembering:

     condition
  if           else
a                   b 

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#7

Ternary conditional operator simply allows testing a condition in a single line replacing the multiline if-else making the code compact.

Syntax :

[on_true] if [expression] else [on_false]

1- Simple Method to use ternary operator:

# Program to demonstrate conditional operator
a, b = 10, 20
# Copy value of a in min if a < b else copy b
min = a if a < b else b
print(min)  # Output: 10

2- Direct Method of using tuples, Dictionary, and lambda:

# Python program to demonstrate ternary operator
a, b = 10, 20
# Use tuple for selecting an item
print( (b, a) [a < b] )
# Use Dictionary for selecting an item
print({True: a, False: b} [a < b])
# lamda is more efficient than above two methods
# because in lambda  we are assure that
# only one expression will be evaluated unlike in
# tuple and Dictionary
print((lambda: b, lambda: a)[a < b]()) # in output you should see three 10

3- Ternary operator can be written as nested if-else:

# Python program to demonstrate nested ternary operator
a, b = 10, 20
print ("Both a and b are equal" if a == b else "a is greater than b"
        if a > b else "b is greater than a")

Above approach can be written as:

# Python program to demonstrate nested ternary operator
a, b = 10, 20
if a != b:
    if a > b:
        print("a is greater than b")
    else:
        print("b is greater than a")
else:
    print("Both a and b are equal") 
# Output: b is greater than a

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#8

More a tip than an answer (don't need to repeat the obvious for the hundreth time), but I sometimes use it as a oneliner shortcut in such constructs:

if conditionX:
    print('yes')
else:
    print('nah')

, becomes:

print('yes') if conditionX else print('nah')

Some (many :) may frown upon it as unpythonic (even, ruby-ish :), but I personally find it more natural - i.e. how you'd express it normally, plus a bit more visually appealing in large blocks of code.