Python’s Disappearing Long Type
Note: everything below refers to the default (missionary position) C implementation of Python.
If you are converting Python code from Python 2 to Python 3, you might notice that the conversion tool transforms any uses of long() into int(). If that confuses you, this post will hopefully make it clear.
Before Python 2.2, there was a clear distinction between two of the Python numerical types, the int type and the Python long type.
Firstly, Python’s int type was implemented as a signed long. So a Python int takes 32 bits of memory, which while not as efficient as some really optimised approach using shorter types, is still very fast indeed.
Secondly, Python’s long type is an integer of unlimited size (well until you run of RAM - which would be an unrealistically massive number not useful for anything).
Python’s long type does not map directly to a C type, it is a custom type implemented in the Python source code somewhere which I guess uses a C struct or whatever. As you might imagine, using the Python long type is significantly more RAM intensive and slower than the Python int type, but in reality it is rarely a problem (see below).
Hans Fangohr did a little performance testing and found that Python’s long type is about three times slower than the Python’s int type.
Unified ints were brought in for Python 2.2. This starts off as a Python int but transforms magically to a Python long if it needs to. Here is how it works in Python 2.2 to 2.7:
>>> import sys >>> sys.maxsize 9223372036854775807 >>> type(sys.maxsize) <type 'int'> >>> sys.maxsize + 1 9223372036854775808L >>> type(sys.maxsize + 1) <type 'long'> >>> long <type 'long'>
Note that when we add 1 to sys.maxsize, the result has an L suffix to denote it is a Python long and no longer a 32 bit number.
In Python 3, it works in a similar the way, however the fact you are no longer using a 32 bit type is now completely hidden away from the user:
>>> import sys >>> sys.maxsize 9223372036854775807 >>> type(sys.maxsize) <class 'int'> >>> sys.maxsize + 1 9223372036854775808 >>> type(sys.maxsize + 1) <class 'int'> >>> long Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'long' is not defined
This time, when we add 1 to sys.maxsize, the result has no L suffix; trying to call the long constructor function causes an exception because it does not exist anymore in Python 3.
Of course, the fun of Python is that being a high level language, we normally don’t really care as long a we get a number; this is it rightly got changed it to be one unified type.
One might design a high performance application not to use the Python long type if it turns out to be a bottleneck. However, normally you would have other bigger insurmountable bottlenecks in your software/hardware/network stack so you don’t care about this.
However, if you are working on a multi-language project, especially if you are using Python alongside a lower level language like C, then it is useful to know what is going on underneath the Python types.
The Python float type is implemented as a C double. This doesn’t change across versions. Several other numeric types are available in Python of course.
So if you see long being converted to int by the 2to3 conversion tool, now you know why.
Image Credit: The Spiderbot by Raikoh