What are the implications of running python with the optimize flag?
Question:
What does Python do differently when running with the -O (optimize) flag?
Answers:
assert statements are completely eliminated, as are statement blocks of the form if __debug__: ... (so you can put your debug code in such statements blocks and just run with -O to avoid that debug code).
With -OO, in addition, docstrings are also eliminated.
From the docs:
- You can use the
-O or -OO switches on the Python command to reduce the size of a compiled module. The -O switch removes assert statements, the -OO switch removes both assert statements and __doc__ strings. Since some programs may rely on having these available, you should only use this option if you know what you’re doing. “Optimized” modules have an opt- tag and are usually smaller. Future releases may change the effects of optimization.
- A program doesn’t run any faster when it is read from a
.pyc file than when it is read from a .py file; the only thing that’s faster about .pyc files is the speed with which they are loaded.
So in other words, almost nothing.
From What does the -O flag do?
It somewhat depends on the Python
version. To find out precisely what it
does, search the source code for
Py_OptimizeFlag. In 2.5, it
- causes the interpreter to load .pyo files, not .pyc files (in .zip files,
just makes .pyo preferred over .pyc)
- causes
__debug__ to have a value of 0
- ignores assert statements in source code
- treats
__debug__ statically as being 0
- causes the byte code generator to save .pyo files, not .pyc
As answered in python optimization mode:
python -O does the following currently:
- completely ignores asserts
- sets the special builtin name
__debug__ to False (which by default is True)
and when called as python -OO
- removes docstrings from the code
I don’t know why everyone forgets to mention the __debug__ issue; perhaps it is because I’m the only one using it 🙂 An if __debug__ construct creates no bytecode at all when running under -O, and I find that very useful.
What does Python do differently when running with the -O (optimize) flag?
assert statements are completely eliminated, as are statement blocks of the form if __debug__: ... (so you can put your debug code in such statements blocks and just run with -O to avoid that debug code).
With -OO, in addition, docstrings are also eliminated.
From the docs:
- You can use the
-Oor-OOswitches on the Python command to reduce the size of a compiled module. The-Oswitch removes assert statements, the-OOswitch removes both assert statements and __doc__ strings. Since some programs may rely on having these available, you should only use this option if you know what you’re doing. “Optimized” modules have anopt-tag and are usually smaller. Future releases may change the effects of optimization.- A program doesn’t run any faster when it is read from a
.pycfile than when it is read from a.pyfile; the only thing that’s faster about.pycfiles is the speed with which they are loaded.
So in other words, almost nothing.
From What does the -O flag do?
It somewhat depends on the Python
version. To find out precisely what it
does, search the source code for
Py_OptimizeFlag. In 2.5, it
- causes the interpreter to load .pyo files, not .pyc files (in .zip files,
just makes .pyo preferred over .pyc)- causes
__debug__to have a value of 0- ignores assert statements in source code
- treats
__debug__statically as being 0- causes the byte code generator to save .pyo files, not .pyc
As answered in python optimization mode:
python -O does the following currently:
- completely ignores asserts
- sets the special builtin name
__debug__to False (which by default is True)
and when called as python -OO
- removes docstrings from the code
I don’t know why everyone forgets to mention the __debug__ issue; perhaps it is because I’m the only one using it 🙂 An if __debug__ construct creates no bytecode at all when running under -O, and I find that very useful.