python assignment fails in else part of conditional operation

Question

Why assignment fails at else part ? I was trying to increment two variables based on a condition in below one line .

>>> a=0
>>> b=0
>>> a+=1 if True else b
>>> a
>>> 1
>>> a if True else b+=1
  File "<input>", line 1
SyntaxError: can't assign to conditional expression
>>> a if False else b+=1
  File "<input>", line 1
SyntaxError: can't assign to conditional expression
>>> a+=1 if False else b
>>> a
>>> 1
>>> a+=1 if True else b+=1
  File "<input>", line 1
    a+=1 if True else b+=1
                        ^
SyntaxError: invalid syntax

The syntax is expression if condition else expression — b+=1 is not an expression. — khelwood
– khelwood, Commented Feb 16, 2018 at 10:26
@khelwood: Yes, but i wondered why it worked a+=1 if False else b. But the accepted answer from @liliscent clarified it. — Rilwan
– Rilwan, Commented Feb 16, 2018 at 10:38

llllllllll · Accepted Answer · 2018-02-16 10:24:41Z

4

a+=1 if True else b is parsed as

a += (1 if True else b)

In Python, assignment is not an expression.

answered Feb 16, 2018 at 10:24

llllllllll

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Rilwan Over a year ago

Thanks for the clarification, Just confused when a+=1 worked at if part. But actually worked because of parsing as you explained.

Hara · Accepted Answer · 2018-02-16 10:56:35Z

4

In python, in a single line, the right side of assignment operator you can't use assignment again.

for example a = b + (c = d), cannot be done in python.

Referring to above example again:

Valid operations

b = 10
a = 20

a+=1 if True else b  # this is valid.
a+=1 if False else b  # this is valid.

Invalid operations

a if True else b+=1  # this is invalid.
a if False else b+=1  # this is invalid.
a+=1 if True else b+=1  # this is invalid.
t = a if True else b=1  # this is invalid.

In these statements, if we try to print a if True else b would have printed b value. But above code, trying to use assignment in the expression, it will not be allowed in python.

This kind of syntax is allowed in C language, but not in python. For example, in C-language we can write like

if (a=10): printf("%d", a);

Will actually does assignment inside the condition to a and pass the condition and executes if part.

In python, it just throws an error if you write code as:

if a=10: print a

one can only write in python like

if a==10 : print a

answered Feb 16, 2018 at 10:56

Hara

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1 Comment

Rilwan Over a year ago

Thanks ! The problem was when I read the line a+=1 if True else b as a ternary like if else instead of assignment statement.

Sorin · Accepted Answer · 2018-02-16 10:29:42Z

0

What you want is

if True: a+=1
else: b+=1

answered Feb 16, 2018 at 10:29

Sorin

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1 Comment

Rilwan Over a year ago

Yes, I could. But i was trying to write in one line.

Gal Bashan · Accepted Answer · 2018-02-16 10:32:00Z

-1

The reason is the meaning of the syntax x if cond else y in python. What this syntax mean is not operation_a if condition else operation_b but rather value_a if condition else value_b

This means the correct way of using it is

var = val1 if condition else val2

And not:

var1=val1 if condition else var2=val2

Read more at:details about if else oneliner operator

P.S: using a hack like:

(var1=val1)  if condition else (var2=val2)

Might work, but don't use it since it is not pythonic and the return value of this statement is not obvious at first glance ("explicit is better then implicit")

answered Feb 16, 2018 at 10:32

Gal Bashan

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