Python 实用宝典

Question 1

I have read the links below, but it doesn’t address my question.
Does Python have a ternary conditional operator? (the question is about condensing if-else statement to one line)

Is there an easier way of writing an if-elif-else statement so it fits on one line?
For example,

if expression1:
   statement1
elif expression2:
   statement2
else:
   statement3

Or a real-world example:

if i > 100:
    x = 2
elif i < 100:
    x = 1
else:
    x = 0

I just feel if the example above could be written the following way, it could look like more concise.

x=2 if i>100 elif i<100 1 else 0 [WRONG]

Question 2

No, it’s not possible (at least not with arbitrary statements), nor is it desirable. Fitting everything on one line would most likely violate PEP-8 where it is mandated that lines should not exceed 80 characters in length.

It’s also against the Zen of Python: “Readability counts”. (Type import this at the Python prompt to read the whole thing).

You can use a ternary expression in Python, but only for expressions, not for statements:

>>> a = "Hello" if foo() else "Goodbye"

Edit:

Your revised question now shows that the three statements are identical except for the value being assigned. In that case, a chained ternary operator does work, but I still think that it’s less readable:

>>> i=100
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
0
>>> i=101
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
2
>>> i=99
>>> a = 1 if i<100 else 2 if i>100 else 0
>>> a
1

Question 3

If you only need different expressions for different cases then this may work for you:

expr1 if condition1 else expr2 if condition2 else expr

For example:

a = "neg" if b<0 else "pos" if b>0 else "zero"

Question 4

Just nest another if clause in the else statement. But that doesn’t make it look any prettier.

>>> x=5
>>> x if x>0 else ("zero" if x==0 else "invalid value")
5
>>> x = 0
>>> x if x>0 else ("zero" if x==0 else "invalid value")
'zero'
>>> x = -1
>>> x if x>0 else ("zero" if x==0 else "invalid value")
'invalid value'

Question 5

Despite some other answers: YES it IS possible:

if expression1:
   statement1
elif expression2:
   statement2
else:
   statement3

translates to the following one liner:

statement1 if expression1 else (statement2 if expression2 else statement3)

in fact you can nest those till infinity. Enjoy ;)

Question 6

You can optionally actually use the get method of a dict:

x = {i<100: -1, -10<=i<=10: 0, i>100: 1}.get(True, 2)

You don’t need the get method if one of the keys is guaranteed to evaluate to True:

x = {i<0: -1, i==0: 0, i>0: 1}[True]

At most one of the keys should ideally evaluate to True. If more than one key evaluates to True, the results could seem unpredictable.

Question 7

There’s an alternative that’s quite unreadable in my opinion but I’ll share anyway just as a curiosity:

x = (i>100 and 2) or (i<100 and 1) or 0

More info here: https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not

Question 8

if i > 100:
    x = 2
elif i < 100:
    x = 1
else:
    x = 0

If you want to use the above-mentioned code in one line, you can use the following:

x = 2 if i > 100 else 1 if i < 100 else 0

On doing so, x will be assigned 2 if i > 100, 1 if i < 100 and 0 if i = 100

Question 9

It also depends on the nature of your expressions. The general advice on the other answers of “not doing it” is quite valid for generic statements and generic expressions.

But if all you need is a “dispatch” table, like, calling a different function depending on the value of a given option, you can put the functions to call inside a dictionary.

Something like:

def save(): 
   ...
def edit():
   ...
options = {"save": save, "edit": edit, "remove": lambda : "Not Implemented"}

option = get_input()
result = options[option]()

Instead of an if-else:

if option=="save":
    save()
...

Question 10

People have already mentioned ternary expressions. Sometimes with a simple conditional assignment as your example, it is possible to use a mathematical expression to perform the conditional assignment. This may not make your code very readable, but it does get it on one fairly short line. Your example could be written like this:

x = 2*(i>100) | 1*(i<100)

The comparisons would be True or False, and when multiplying with numbers would then be either 1 or 0. One could use a + instead of an | in the middle.

Question 11

The ternary operator is the best way to a concise expression. The syntax is variable = value_1 if condition else value_2. So, for your example, you must apply the ternary operator twice:

i = 23 # set any value for i
x = 2 if i > 100 else 1 if i < 100 else 0

Question 12

You can use nested ternary if statements.

# if-else ternary construct
country_code = 'USA'
is_USA = True if country_code == 'USA' else False
print('is_USA:', is_USA)

# if-elif-else ternary construct
# Create function to avoid repeating code.
def get_age_category_name(age):
    age_category_name = 'Young' if age <= 40 else ('Middle Aged' if age > 40 and age <= 65 else 'Senior')
    return age_category_name

print(get_age_category_name(25))
print(get_age_category_name(50))
print(get_age_category_name(75))

Python 实用宝典

将if-elif-else语句放在一行上吗？

问题：将if-elif-else语句放在一行上吗？

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