How to check single line if else if else condition in Python

Question

I have three different answers for holiday_type

holiday_type = Approved
holiday_type = Confirmed
holiday_type = both

Python Code:

 result = ['confirm','validate'] if holiday_type == 'both' else ['confirm'] if holiday_type == 'Confirmed' else ['validate']

Result:

['validate']                // Approved
['confirm']                 // Confirmed
['confirm', 'validate']     // both

I can't understand how to compile this if else statement: which one first which one second. Can you please explain how to compile this condition flow.

Off cause is this pythonic............ but I just want to know flow of this execution. because for loop execution start at the end. — Jaykumar Patel
– Jaykumar Patel, Commented Apr 9, 2015 at 11:16
Hint: result = ['confirm','validate'] if holiday_type == 'both' else (['confirm'] if holiday_type == 'Confirmed' else ['validate']) - you can easily see two nested similar statements of true_val if condition else false_val form. — Łukasz Rogalski
– Łukasz Rogalski, Commented Apr 9, 2015 at 11:16
I say execution flow start at the end (right to left) so in this case how is this become nested — Jaykumar Patel
– Jaykumar Patel, Commented Apr 9, 2015 at 11:17
@S͢kyD͢ream: No: there's no simple left-to-right or right-to-left evaluation order here. The evaluation order of the conditional expression true_expr if condition else false_expr is condition and then one of true_expr or false_expr is evaluated, as necessary. — PM 2Ring
– PM 2Ring, Commented Apr 9, 2015 at 11:24

Daniel Roseman · Accepted Answer · 2015-04-09 11:16:30Z

3

Don't do this. Readability counts.

if holiday_type == 'both':
    result = ['confirm','validate']
elif  holiday_type == 'Confirmed':
    result = ['confirm']
else:
    result = ['validate']

answered Apr 9, 2015 at 11:16

Daniel Roseman

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1 Comment

ideasman42 Over a year ago

this doesn't answer the question (while strange use of ternary operations - is a valid cryt). Sometimes you read other peoples strange code so you need to know how it works exactly before refactoring it.

haccks · Accepted Answer · 2015-04-09 11:35:02Z

2

Statement

<expression1> if <condition> else <expression2>

first evaluates the condition; if it returns True, expression1 will be evaluated to give the result, otherwise expression2.

In

 result = ['confirm','validate'] if holiday_type == 'both' else ['confirm'] if holiday_type == 'Confirmed' else ['validate']

['confirm'] if holiday_type == 'Confirmed' else ['validate'] is expression2 and it will be evaluated if holiday_type == 'both' is evaluated to False.

answered Apr 9, 2015 at 11:35

haccks

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Comments

hyades · Accepted Answer · 2015-04-09 11:19:48Z

0

This is a nested if-else

generally speaking - x if cond else y is the normal syntax or a type of statement

now here x and y can be independent set of statements

You can expand them.

Like in your case y is the same statement -> x if cond else y So it becomes

x if cond else x1 if cond2 else y1

answered Apr 9, 2015 at 11:19

hyades

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Comments

Open AI - Opting Out · Accepted Answer · 2015-04-09 11:32:31Z

0

result = (['confirm','validate'] if holiday_type == 'both' else
          ['confirm'] if holiday_type == 'Confirmed' else
          ['validate'])

answered Apr 9, 2015 at 11:32

Open AI - Opting Out

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Comments

Jaykumar Patel · Accepted Answer · 2015-06-01 13:30:43Z

0

If th expression is different for different cases, this would be works,

expr1 if condition1 else expr2 if condition2 else expr

For example:

>>> a = -5
>>> "negative" if a<0 else "zero" if a==0 else "positive"
'negative'
>>> a = 5
>>> "negative" if a<0 else "zero" if a==0 else "positive"
'positive'
>>> a = 0
>>> "Negative" if a<0 else "zero" if a==0 else "positive"
'zero'

Question Example:

result = ['confirm','validate'] if holiday_type == 'both' else ['confirm'] if holiday_type == 'Confirmed' else ['validate']

answered Jun 1, 2015 at 13:30

Jaykumar Patel

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Comments

superbeck · Accepted Answer · 2016-01-12 20:18:05Z

0

I'm going to borrow from others but give a more generic explanation of how to parse these oneline if statements by translating it to multiple lines.

expr1 if condition1 else expr2 if condition2 else expr

translates to

if condition1:
  expr1
elif condition2:
  expr2
else:
  expr

Keep in mind this same style of syntax is used for list comprehensions and other areas of python.

answered Jan 12, 2016 at 20:18

superbeck

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