Why does this print this!

[UnspeC5d]

x, y = 20, 60
y, x, y = x, y-10, x+10
print(x, y)

It returns 50, 30 which means the second line is as good as

x, y = y-10, x+10

Why is the first assignment useless?

 

Won't python first assign y = x, then do the operations and then finally gets reassigned?

[C2dan88]

Because you are overwriting the variables. Inline assignment is the same as this

x = 20
y = 60

y = x
x = y-10
y = x+10 <--- this will overwites the first assignment y=x

 

[Guest]

Why are you assigning y to x, but then assigning y to x + 10?

It's printing x = 60 - 10, y = 30 + 10, hence the 50 30. You're trying to assign it twice.

[UnspeC5d]

2 minutes ago, C2dan88 said:

Because you are overwriting the variables. Inline assignment is the same as this

x = 20
y = 60

y = x
x = y-10
y = x+10 <--- this will overwites the first assignment y=x

 

So the interpreter ignores the first assignment or...? Thanks a lot for the reply

[UnspeC5d]

1 minute ago, lewdicrous said:

Why are you assigning y to x, but then assigning y to x + 10?

It's printing x = 60 - 10, y = 30 + 10, hence the 50 30.

It's a question in my CS textbook

[Guest]

4 minutes ago, UnspeC5d said:

So the interpreter ignores the first assignment or...? Thanks a lot for the reply

I think it's reassigning it 'cause you're trying to assign it twice on the same line/at the same time, so it overwrites it like how @C2dan88 illustrated.

[jaslion]

4 minutes ago, UnspeC5d said:

So the interpreter ignores the first assignment or...? Thanks a lot for the reply

No a computer works line by line.

 

x = 20

y = 60

 

y = x //y is now 20

x = y-10 //x is now 10

y = x+10 //y is now 20

 

Anything you assign to a variable overwrites the previous assigned value.

[UnspeC5d]

1 hour ago, jaslion said:

No a computer works line by line.

 

x = 20

y = 60

 

y = x //y is now 20

x = y-10 //x is now 10

y = x+10 //y is now 20

 

Anything you assign to a variable overwrites the previous assigned value.

Doesn't work that way when I run it 

[jaslion]

5 minutes ago, UnspeC5d said:

Doesn't work that way when I run it 

Then there is something wrong with the way you do it as when I run that exact piece of code I typed I get x = 10, y = 20 as the result should be.

 

[0x21]

The byte code makes it quite clear:

  1           0 LOAD_CONST               0 ((20, 60))
              2 UNPACK_SEQUENCE          2
              4 STORE_NAME               0 (x)
              6 STORE_NAME               1 (y)

  2           8 LOAD_NAME                0 (x)
             10 LOAD_NAME                1 (y)
             12 LOAD_CONST               1 (10)
             14 BINARY_SUBTRACT
             16 LOAD_NAME                0 (x)
             18 LOAD_CONST               1 (10)
             20 BINARY_ADD
             22 ROT_THREE
             24 ROT_TWO
             26 STORE_NAME               1 (y)
             28 STORE_NAME               0 (x)
             30 STORE_NAME               1 (y)
             32 LOAD_CONST               2 (None)
             34 RETURN_VALUE

This is perfectly normal and what we expect. When doing an assignment, it will fully evaluate the right hand side before storing/setting the variables.

 

The code that @jaslion is running is not in any way equivalent, hence the different output.