Why does semicolon in python make a different result

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12
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I found a strange question about semicolon ";" in python.

>>> x=20000;y=20000
>>> x is y
True
>>> x=20000
>>> y=20000
>>> x is y
False
>>> x=20000;
>>> y=20000
>>> x is y
False

Why is it that the first test returns True and the other two return False?

Python Version: 3.6.5

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

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up vote
12
down vote

favorite

I found a strange question about semicolon ";" in python.

>>> x=20000;y=20000
>>> x is y
True
>>> x=20000
>>> y=20000
>>> x is y
False
>>> x=20000;
>>> y=20000
>>> x is y
False

Why is it that the first test returns True and the other two return False?

Python Version: 3.6.5

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

New contributor

add a commentÂ |Â

up vote
12
down vote

favorite

I found a strange question about semicolon ";" in python.

>>> x=20000;y=20000
>>> x is y
True
>>> x=20000
>>> y=20000
>>> x is y
False
>>> x=20000;
>>> y=20000
>>> x is y
False

Why is it that the first test returns True and the other two return False?

Python Version: 3.6.5

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

New contributor

I found a strange question about semicolon ";" in python.

>>> x=20000;y=20000
>>> x is y
True
>>> x=20000
>>> y=20000
>>> x is y
False
>>> x=20000;
>>> y=20000
>>> x is y
False

Why is it that the first test returns True and the other two return False?

Python Version: 3.6.5

python

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

New contributor

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

New contributor

edited 38 mins ago

NaruS

6812

edited 38 mins ago

NaruS

6812

edited 38 mins ago

NaruS

6812

asked 1 hour ago

longshuai ma

614

New contributor

asked 1 hour ago

longshuai ma

614

asked 1 hour ago

longshuai ma

614

New contributor

longshuai ma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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2 Answers
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up vote
15
down vote

In the interactive interpreter, the first semi-colon line is read and evaluated in one pass. As such, the interpreter recognizes that 20000 is the same immutable int value in each assignment, and so can (it doesn't have to, but does) make x and y references to the same object.

The important point is that this is simply an optimization that the interactive interpreter chooses to make; it's not something guaranteed by the language or some special property of the ; that joins two statements into one.

In the following two examples, by the time y=20000 is read and evaluated, x=20000 (with or without the semi-colon) has already ben evaluated and forgotten. Since 20000 isn't in the range (-5 to 257) of pre-allocated int values, CPython doesn't try to find another instance of 20000 already in memory; it just creates a new one for y.

answered 1 hour ago

chepner

229k27214311

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

add a commentÂ |Â

up vote
1
down vote

The is operator checks whether two values are the same object in memory. It's not meant to be used for checking for equality. For what is worth, you could consider the fact that it sometimes returns True and sometimes `False+ just to be a matter of luck.

For example, the results are different in an interactive session and in a standalone program:

$ cat test.py
x = 200000; y = 200000
print(x is y)

xx = 200000
yy = 200000
print(xx is yy)

$ python test.py
True
True

Or you have this other example:

>>> x = 50 + 50; y = 50 + 50
>>> x is y
True
>>> x = 5000 + 5000; y = 5000 + 5000
>>> x is y
False

This happens because the interpreter caches small numbers so they are always the same object, but it doesn't for large numbers, so both additions in the second case create a new 10000 object. It has nothing to do with the semicolon.

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

add a commentÂ |Â

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2 Answers
2

active

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votes

2 Answers
2

active

oldest

votes

up vote
15
down vote

answered 1 hour ago

chepner

229k27214311

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

add a commentÂ |Â

up vote
15
down vote

answered 1 hour ago

chepner

229k27214311

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

add a commentÂ |Â

up vote
15
down vote

answered 1 hour ago

chepner

229k27214311

answered 1 hour ago

chepner

229k27214311

answered 1 hour ago

chepner

229k27214311

answered 1 hour ago

chepner

229k27214311

answered 1 hour ago

chepner

229k27214311

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

add a commentÂ |Â

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

Thanks, i understand. then i also try x,y=20000,20000, the "x is y" returns true also. so i think "x=20000;y=20000" may optimiz to "x,y=20000,20000".
â€“Â longshuai ma
43 mins ago

Not directly, but again, because the commands are in one "batch" of input, the interpreter/compiler has more opportunity to look for optimizations. The tuple created on the right-hand side consists of the same literal repeated twice, so the tuple can be created with two references to the same object, rather than allocating two separate objects with the same value.
â€“Â chepner
23 mins ago

add a commentÂ |Â

up vote
1
down vote

For example, the results are different in an interactive session and in a standalone program:

$ cat test.py
x = 200000; y = 200000
print(x is y)

xx = 200000
yy = 200000
print(xx is yy)

$ python test.py
True
True

Or you have this other example:

>>> x = 50 + 50; y = 50 + 50
>>> x is y
True
>>> x = 5000 + 5000; y = 5000 + 5000
>>> x is y
False

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

add a commentÂ |Â

up vote
1
down vote

For example, the results are different in an interactive session and in a standalone program:

$ cat test.py
x = 200000; y = 200000
print(x is y)

xx = 200000
yy = 200000
print(xx is yy)

$ python test.py
True
True

Or you have this other example:

>>> x = 50 + 50; y = 50 + 50
>>> x is y
True
>>> x = 5000 + 5000; y = 5000 + 5000
>>> x is y
False

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

add a commentÂ |Â

up vote
1
down vote

For example, the results are different in an interactive session and in a standalone program:

$ cat test.py
x = 200000; y = 200000
print(x is y)

xx = 200000
yy = 200000
print(xx is yy)

$ python test.py
True
True

Or you have this other example:

>>> x = 50 + 50; y = 50 + 50
>>> x is y
True
>>> x = 5000 + 5000; y = 5000 + 5000
>>> x is y
False

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

For example, the results are different in an interactive session and in a standalone program:

$ cat test.py
x = 200000; y = 200000
print(x is y)

xx = 200000
yy = 200000
print(xx is yy)

$ python test.py
True
True

Or you have this other example:

>>> x = 50 + 50; y = 50 + 50
>>> x is y
True
>>> x = 5000 + 5000; y = 5000 + 5000
>>> x is y
False

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

edited 31 mins ago

answered 59 mins ago

Roberto Bonvallet

19.9k53349

answered 59 mins ago

Roberto Bonvallet

19.9k53349

answered 59 mins ago

Roberto Bonvallet

19.9k53349

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

add a commentÂ |Â

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

How come can one consider it to be a matter of luck while it isn't at all?
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
47 mins ago

and when it will return False?
â€“Â mad_
43 mins ago

@IÃ…ÂŸÃ„Â±kKaplan You're right that it isn't. What I mean is that it's not a behavior you can rely on. In some sense it's a matter of luck that the interpreter chooses to the optimization that Chepner mentioned, and also that the example was tested in the interactive console (I tried writing a standalone program and the results are different).
â€“Â Roberto Bonvallet
34 mins ago

@mad_ I expanded my answer with an example where two expressions with equal result separated by semicolons make the is operator return False.
â€“Â Roberto Bonvallet
25 mins ago

There isn't an inconsistency though, you may want to stay away from the immutable built-ins to don't think about interpreter optimizations that much but, for custom classes it is always accurate and something that can be relied upon. If not; I'd like to learn where it can return incorrect result in a custom class.
â€“Â IÃ…ÂŸÃ„Â±k Kaplan
22 mins ago

add a commentÂ |Â

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