Python是否具有三元条件运算符?

If Python does not have a ternary conditional operator, is it possible to simulate one using other language constructs?


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 assignment statements or pass or other statements within a conditional expression: >>> pass if False else x = 3 File "<stdin>", line 1 pass if False else x = 3 ^ SyntaxError: invalid syntax You can, however, use conditional expressions to assign a variable like so: x = a if True else b Think of the conditional expression as switching between two values. It is very useful when you're in a 'one value or another' situation, it but doesn't do much else. If you need to use statements, you have to use a normal if statement instead of a conditional expression. 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?

<expression 1> if <condition> else <expression 2> a = 1 b = 2 1 if a > b else -1 # Output is -1 1 if a > b else -1 if a < b else 0 # Output is -1

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.

An operator for a conditional expression in Python was added in 2006 as part of Python Enhancement Proposal 308. Its form differ from common ?: operator and it's: <expression1> if <condition> else <expression2> which is equivalent to: if <condition>: <expression1> else: <expression2> Here is an example: result = x if a > b else y Another syntax which can be used (compatible with versions before 2.5): result = (lambda:y, lambda:x)[a > b]() where operands are lazily evaluated. Another way is by indexing a tuple (which isn't consistent with the conditional operator of most other languages): result = (y, x)[a > b] or explicitly constructed dictionary: result = {True: x, False: y}[a > b] Another (less reliable), but simpler method is to use and and or operators: result = (a > b) and x or y however this won't work if x would be False. A possible workaround is to make x and y lists or tuples as in the following: result = ((a > b) and [x] or [y])[0] or: result = ((a > b) and (x,) or (y,))[0] If you're working with dictionaries, instead of using a ternary conditional, you can take advantage of get(key, default), for example: shell = os.environ.get('SHELL', "/bin/sh") Source: ?: in Python at Wikipedia

Unfortunately, the (falseValue, trueValue)[test] solution doesn't have short-circuit behaviour; thus both falseValue and trueValue are evaluated regardless of the condition. This could be suboptimal or even buggy (i.e. both trueValue and falseValue could be methods and have side-effects). One solution to this would be (lambda: falseValue, lambda: trueValue)[test]() (execution delayed until the winner is known ;)), but it introduces inconsistency between callable and non-callable objects. In addition, it doesn't solve the case when using properties. And so the story goes - choosing between 3 mentioned solutions is a trade-off between having the short-circuit feature, using at least Зython 2.5 (IMHO not a problem anymore) and not being prone to "trueValue-evaluates-to-false" errors.

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

For Python 2.5 and newer there is a specific syntax: [on_true] if [cond] else [on_false] In older Pythons a ternary operator is not implemented but it's possible to simulate it. cond and on_true or on_false Though, there is a potential problem, which if cond evaluates to True and on_true evaluates to False then on_false is returned instead of on_true. If you want this behavior the method is OK, otherwise use this: {True: on_true, False: on_false}[cond is True] # is True, not == True which can be wrapped by: def q(cond, on_true, on_false) return {True: on_true, False: on_false}[cond is True] and used this way: q(cond, on_true, on_false) It is compatible with all Python versions.

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

Does Python have a ternary conditional operator? Yes. From the grammar file: test: or_test ['if' or_test 'else' test] | lambdef The part of interest is: or_test ['if' or_test 'else' test] So, a ternary conditional operation is of the form: expression1 if expression2 else expression3 expression3 will be lazily evaluated (that is, evaluated only if expression2 is false in a boolean context). And because of the recursive definition, you can chain them indefinitely (though it may considered bad style.) expression1 if expression2 else expression3 if expression4 else expression5 # and so on A note on usage: Note that every if must be followed with an else. People learning list comprehensions and generator expressions may find this to be a difficult lesson to learn - the following will not work, as Python expects a third expression for an else: [expression1 if expression2 for element in iterable] # ^-- need an else here which raises a SyntaxError: invalid syntax. So the above is either an incomplete piece of logic (perhaps the user expects a no-op in the false condition) or what may be intended is to use expression2 as a filter - notes that the following is legal Python: [expression1 for element in iterable if expression2] expression2 works as a filter for the list comprehension, and is not a ternary conditional operator. Alternative syntax for a more narrow case: You may find it somewhat painful to write the following: expression1 if expression1 else expression2 expression1 will have to be evaluated twice with the above usage. It can limit redundancy if it is simply a local variable. However, a common and performant Pythonic idiom for this use-case is to use or's shortcutting behavior: expression1 or expression2 which is equivalent in semantics. Note that some style-guides may limit this usage on the grounds of clarity - it does pack a lot of meaning into very little syntax.

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'

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

a if condition else b Just memorize this pyramid if you have trouble remembering: condition if else a b

you can do this :- [condition] and [expression_1] or [expression_2] ; Example:- print(number%2 and "odd" or "even") This would print "odd" if the number is odd or "even" if the number is even. The result :- If condition is true exp_1 is executed else exp_2 is executed. Note :- 0 , None , False , emptylist , emptyString evaluates as False. And any data other than 0 evaluates to True. Here's how it works: if the condition [condition] becomes "True" then , expression_1 will be evaluated but not expression_2 . If we "and" something with 0 (zero) , the result will always to be fasle .So in the below statement , 0 and exp The expression exp won't be evaluated at all since "and" with 0 will always evaluate to zero and there is no need to evaluate the expression . This is how the compiler itself works , in all languages. In 1 or exp the expression exp won't be evaluated at all since "or" with 1 will always be 1. So it won't bother to evaluate the expression exp since the result will be 1 anyway . (compiler optimization methods). But in case of True and exp1 or exp2 The second expression exp2 won't be evaluated since True and exp1 would be True when exp1 isn't false . Similarly in False and exp1 or exp2 The expression exp1 won't be evaluated since False is equivalent to writing 0 and doing "and" with 0 would be 0 itself but after exp1 since "or" is used, it will evaluate the expression exp2 after "or" . Note:- This kind of branching using "or" and "and" can only be used when the expression_1 doesn't have a Truth value of False (or 0 or None or emptylist [ ] or emptystring ' '.) since if expression_1 becomes False , then the expression_2 will be evaluated because of the presence "or" between exp_1 and exp_2. In case you still want to make it work for all the cases regardless of what exp_1 and exp_2 truth values are, do this :- [condition] and ([expression_1] or 1) or [expression_2] ;

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.

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.

As already answered, yes there is a ternary operator in python: <expression 1> if <condition> else <expression 2> Additional information: If <expression 1> is the condition you can use Short-cirquit evaluation: a = True b = False # Instead of this: x = a if a else b # You could use Short-cirquit evaluation: x = a or b PS: Of course, a Short-cirquit evaluation is not a ternary operator but often the ternary is used in cases where the short circuit would be enough.

Many programming languages derived from C usually have the following syntax of ternary conditional operator: <condition> ? <expression1> : <expression2> At first, the Python 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' Hope this helps.

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)

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 most succinct way - see 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 interp treats that as False in logic expressions. The interp 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 dabler. But the more you learn and understand the mechanics of the underlying system, the more you appreciate it.

I find cumbersome the default python syntax val = a if cond else b, 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 it's way clearer to me

if variable is defined and you want to check if it has value you can just a or b def test(myvar=None): # shorter than: print myvar if myvar else "no Input" print myvar or "no Input" test() test([]) test(False) test('hello') test(['Hello']) test(True) will output no Input no Input no Input hello ['Hello'] True

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

is_spacial=True if gender = "Female" else (True if age >= 65 else False) ** it can be nested as your need. best of luck **