Supporting Python 2 and 3 without 2to3 conversion

Although the official documentation for Python 3 discourages writing code for both Python 2 and Python 3, in some cases it is desirable. Especially if you can drop support for Python 2.5 and earlier, since Python 2.6 introduces quite a lot of forwards compatibility.

It’s possible to make the same code run under earlier versions of Python as well, but then you start getting into the “contorted” writing style the Python 3 documentation mentions. I’ll take up tricks to do this and the six module I mention at the end of this chapter will help a lot. It has been done even for some quite big projects, but I would in general not recommend it for a large project. For small projects or parts of bigger projects, for example bootstrapping scripts, supporting old versions of Python without using 2to3 is often the best solution.

Python 2.7 has some small improvements on Python 3 compatibility, but it’s likely that if you want to run the same code under both Python 2 and Python 3 you will have to support Python 2.6 for some time to come.

Many of the changes you need will be done by 2to3, so to start converting your code you actually want to first run 2to3 on your code and make your code run under Python 3. It is generally easier, or at least less monotonous, to introduce Python 2 compatibility in Python 3 code, than to introduce Python 3 compatibility in Python 2 code.

Once you have the project running under Python 3, try to run it under Python 2.6. At this stage you may run into syntax errors. They should come from only changes in the print statement. Once you have fixed them you can fix the remaining errors and then lastly you do the same for earlier versions of Python, if you need to support them as well.

Supporting the print() function

One of the major causes of syntax errors is the change of print from a statement to a function. The simple cases are not problematic, you can simply put parentheses around the text that should be printed. The following will print exactly the same in all versions of Python:

>>> print("This works in all versions of Python!")
This works in all versions of Python!

However, if you use any more advanced feature of print you either end up with a syntax error or not printing what you intended. Python 2’s trailing comma has in Python 3 become a parameter, so if you use trailing commas to avoid the newline after a print, this will in Python 3 look like print('Text to print', end=' ') which is a syntax error under Python 2.

Under Python 2.6 there is a __future__ import to make print into a function. So to avoid any syntax errors and other differences you should start any file where you use print() with from __future__ import print_function. The __future__ import only works under Python 2.6 and later, so for Python 2.5 and earlier you have two options. You can either convert the more complex print to something simpler, or you can use a separate print function that works under both Python 2 and Python 3.

Writing your own print function sounds more complicated than it is. The trick is to use sys.stdout.write() and formatting according to the parameters passed in to the function. However, it is even easier to use the print_() function from the six module that I will present at the end of this chapter.

Handling exceptions

If you in your exception handling need access to the exception itself you need to use an exception variable. In Python 2 the syntax for this is:

>>> try:
...     a = 1/0
... except ZeroDivisionError, e:
...     print e.args[0]
integer division or modulo by zero

However, this syntax is confusing when you need to catch more than one exception. You need to have parentheses around the list of exceptions:

>>> try:
...     a = 1/"Dinsdale"
... except (ZeroDivisionError, TypeError):
...     print "You can't divide by zero or by strings!"
You can't divide by zero or by strings!

If you forget the parentheses only ZeroDivisionError is caught and the raised exception will be stored in a variable named TypeError. That’s not what you expect. Therefore, in Python 3, the syntax has changed to use as instead of a comma, which avoids this mistake. It will also give you a syntax error if you do not have parentheses when catching more than one exception:

>>> try:
...     a = 1/'0'
... except (ZeroDivisionError, TypeError) as e:
...     print(e.args[0])
unsupported operand type(s) for /: 'int' and 'str'

The above syntax also works in Python 2.6 where both the old syntax with a comma and the new syntax using as works. If you need to support Python versions lower than Python 2.6 and you need access to the exception that was raised, you can get that in all versions through the exc_info() function:

>>> import sys
>>> try:
...     a = 1/'0'
... except (ZeroDivisionError, TypeError):
...     e = sys.exc_info()[1]
...     print(e.args[0])
unsupported operand type(s) for /: 'int' and 'str'

Another difference is that Exceptions are no longer iterable. In Python 2 the arguments to the exception was available by iterating over the exception or subscripting the exception.

>>> try:
...    f = 1/0
... except ZeroDivisionError, e:
...    for m in e:
...        print m
...    print e[0]
integer division or modulo by zero
integer division or modulo by zero

In Python 3 you need to use the exception attribute args instead:

>>> try:
...    f = "1" + 1
... except TypeError as e:
...    for m in e.args:
...        print(m)
...    print(e.args[0])
Can't convert 'int' object to str implicitly
Can't convert 'int' object to str implicitly

A message attribute was added to the built-in exceptions in Python 2.5. It was however deprecated already in Python 2.6 and removed in Python 3. Python 2.6 and Python 2.7 will warn you about this when using the -3 flag.

Import errors

One of the big changes is the reorganization of the standard library and as a result the typical errors you will get at this stage are mostly import errors. Getting around it is very easy. You simply try to import from the Python 3 locations and if that fails you import from the Python 2 locations. For modules that have been renamed, you can just import them as the new name.

>>> try:
...     import configparser
... except ImportError:
...     import ConfigParser as configparser

Some of the new modules are mergers of several old modules and in that case the above will not work if you need things from several of the old modules. You can also not do this with modules that have sub-modules. import httplib as http.client is a syntax error. The urllib and urllib2 modules have not only been merged, but reorganized into several sub-modules. So there you need to import each name you use separately. This often means you need to make changes to your code as well. In Python 2 retrieving a web page looks like this:

>>> import urllib
>>> import urlparse
>>> 
>>> url = 'http://docs.python.org/library/'
>>> parts = urlparse.urlparse(url)
>>> parts = parts._replace(path='/3.0'+parts.path)
>>> page = urllib.urlopen(parts.geturl())

After conversion with 2to3 it will look like this:

>>> import urllib.request, urllib.parse, urllib.error
>>> import urllib.parse
>>> 
>>> url = 'http://docs.python.org/library/'
>>> parts = urllib.parse.urlparse(url)
>>> parts = parts._replace(path='/3.0'+parts.path)
>>> page = urllib.request.urlopen(parts.geturl())

Yes, urllib.parse will be imported twice and urllib.error imported even though it isn’t used. That’s how this fixer does it and solving that would be a lot of extra effort, so it imports more than is needed. We need to fix up the code to import the names we use directly instead of the modules they are located in, so the version that runs under both Python 2 and Python 3 ends up like this:

>>> try:
...     from urllib.request import urlopen
...     from urllib.parse import urlparse
... except ImportError:
...     from urlparse import urlparse
...     from urllib import urlopen
...
>>> url = 'http://docs.python.org/library/'
>>> parts = urlparse(url)
>>> parts = parts._replace(path='/3.0'+parts.path)
>>> page = urlopen(parts.geturl())

Integer incompatibilities

There are two big changes in integer handling in Python 3. The first one is that the int and the long types have been merged. That means that you can’t specify that an integer should be long by adding the L suffix any more. 1L is a syntax error in Python 3.

If you have to have an integer being a long in Python 2 and still be compatible with Python 3, the following code will solve it. It defines up a long variable to be the same as the int class under Python 3, and it can then be used explicitly to make sure the integer is a long.

>>> import sys
>>> if sys.version > '3':
...     long = int
>>> long(1)
1L

Another change is that the syntax for octal literals has changed. In Python 2 any number starting with a 0 is an octal, which can be confusing if you decide to format your numbers by starting them with zeros. In Python 3 the syntax has instead changed to the less confusing 0o, similar to 0x used for hex numbers. Starting numbers with 0 is a syntax error to prevent you from trying the old octal syntax by mistake.

Octal is used almost exclusively when setting permissions under Unix, but that in turn is quite a common task. There is an easy way that works under both Python 2 and Python 3: Use the decimal or hex value and put the octal value in a comment:

>>> f = 420 # 644 in octal, 'rw-r--r--'

More bytes, strings and Unicode

It’s no surprise that the trickiest issue we have in supporting Python 2 and Python 3 without 2to3 is handling binary data, just as it is when using 2to3. When we choose not to use 2to3 the problem is made more tricky by making Unicode an issue as well. When using 2to3 to support Python 2 and Python 3, 2to3 will convert any Unicode literals to straight string literals. Without 2to3 we have no such luxury and since the Unicode literal u'' is gone in Python 3 we need to find a portable way to say that we want a Unicode string.

Here, only supporting Python 3.3 will make things much easier for you, because in Python 3.3, the u'' literal is back! In that case you can mostly ignore this section.

But if you need to support Python 3.1 or 3.2, the best way to do this is to make a Unicode string maker function just like the b() function in Common migration problems but for Unicode strings instead of binary bytes. The natural name for this function is of course u(). We would then use b() instead of the b'' literal, and u() instead of the u'' literal.

import sys
if sys.version < '3':
    import codecs
    def u(x):
        return codecs.unicode_escape_decode(x)[0]
else:
    def u(x):
        return x

This will return a unicode object in Python 2:

>>> from makeunicode import u
>>> u('GIF89a')
u'GIF89a'

While it will return a string object in Python 3:

>>> from makeunicode import u
>>> u('GIF89a')
'GIF89a'

Here I use the unicode_escape encoding, because other encodings could fail if you save the file with a different encoding than the one specified in the function. Using unicode_escape is a bit more work that just typing in the characters and saving the file but it will be portable both between different versions of Python and different platforms.

The unicode_escape encoding will convert all the various ways of entering unicode characters. The '\x00' syntax, the '\u0000' and even the '\N{name}' syntax:

>>> from makeunicode import u
>>> print(u('\u00dcnic\u00f6de'))
Ünicöde
>>> print(u('\xdcnic\N{Latin Small Letter O with diaeresis}de'))
Ünicöde

If you only need to support Python 2.6 or later there is also from __future__ import unicode_literals. This will turn all string literals in the file into Unicode literals:

>>> from __future__ import unicode_literals
>>> type("A standard string literal")
<type 'unicode'>
>>> type(b"A binary literal")
<type 'str'>

Both with the __future__ import and the u() function the the binary data type is still called str and the text type is still called unicode under Python 2, while under Python 3 they are called bytes and str.

The best way around this is to define two variables; text_type and binary_type, depending on Python version and then we test against that variable.

>>> from __future__ import unicode_literals
>>> import sys
>>> if sys.version < '3':
...     text_type = unicode
...     binary_type = str
... else:
...     text_type = str
...     binary_type = bytes
>>> isinstance('U\xf1ic\xf6de', text_type)
True

For the handling of binary data you can use the same techniques as discussed in Common migration problems.

Two times three is “six”

There are many many more unusual and sometimes subtle differences between Python 2 and Python 3. Although the techniques mentioned here works for most of them, I definitely recommend you to look at Benjamin Petersons module “six”[1] It contains a PY3 constant to use when checking for the version of Python, and it contains the above mentioned b() and u() functions, although the u() function doesn’t specify an encoding, so you are restricted to using ASCII characters. It also has many helpful constants like text_type and binary_type and a print_() function that works both under Python 2 and Python 3.

It also contains imports of much of the reorganized standard library, so instead of the try/except construct from the beginning of this chapter you can import the module from the six module instead. However it notably doesn’t support the reorganization of the urllib and urllib2 modules, so there you still need to use the try/except import technique.

The six module even contains helpers for unusual problems, like using metaclasses and the attributes of functions, which also have been renamed. Although it requires Python 2.4 or later you can use many of the techniques in it even under earlier versions of Python, if you need to support them.

If you are attempting to support Python 2 and Python 3 without conversion you will definitely find it very helpful.

Footnotes

[1]http://pypi.python.org/pypi/six