CODEWITHSUNDEEP
python
Rilwan
Rilwan

Reputation: 2301

python assignment fails in else part of conditional operation

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

Upvotes: 0

Views: 4737

Answers (4)

Hara
Hara

Reputation: 1502

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

Upvotes: 4

Gal Bashan
Gal Bashan

Reputation: 112

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")

Upvotes: -1

Sorin
Sorin

Reputation: 11968

What you want is

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

Upvotes: 0

llllllllll
llllllllll

Reputation: 16404

a+=1 if True else b is parsed as

a += (1 if True else b)

In Python, assignment is not an expression.

Upvotes: 4

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