Numbers

There have been two major changes in how Python 3 handles numbers: true division replaces truncating division, and the long type was merged into int.

This section describes these changes in detail, along with other, minor ones.

Division

  • Fixer: None
  • Future import: from __future__ import division
  • Prevalence: Common

In Python 2, dividing two integers resulted in an integer:

>>> print 2 / 5
0

This truncating division was inherited from C-based languages, but confused people who don’t know those languages, such as those coming from JavaScript or pure math.

In Python 3, dividing two integers results in a float:

>>> print(2 / 5)
0.4

The // operator, which was added all the way back in Python 2.2, always performs truncating division:

whole_minutes = seconds // 60

The from __future__ import division directive causes the / operator to behave the same in Python 2 as it does in Python 3. We recommend adding it to all modules that use the division operator, so that differences between Python versions are minimal.

When adding the future import, check all divisions in the file and decide if the operator should be changed to //.

Special Methods

To overload the / operator for a class in Python 2, one defined the __div__ special method. With the division change, there are two methods to define:

  • __floordiv__

    Defines the behavior of the // operator.

  • __truediv__

    Defines the behavior of the / operator in Python 3, and in Python 2 when the division future import is in effect.

  • __div__

    Defines the behavior of the / operator in Python 2, when the division future import is not in effect.

    Not used at all in Python 3.

Check all classes that define __div__, and add __floordiv__ and/or __truediv__ as needed. This can be done with a simple alias:

class CustomClass(object):
    def __div__(self, other):
        return _divide(self, other)

    __truediv__ = __div__

Unification of int and long

Python 3 does not have a long type. Instead, int itself allows large values (limited only by available memory); in effect, Python 2’s long was renamed to int.

This change has several consequences.

Removal of the long type

  • Fixer: python-modernize -wnf lib2to3.fixes.fix_long
  • Prevalence: Common

The long builtin no longer exists.

In Python 2, calling int on a number that doesn’t fit in the machine int range would automatically create a long with the appropriate value.

The same automatic conversion to long happened on all operations on int that overflow: for example, 10**50 resulted in a long on most systems.

The range of Python 2’s int was system-dependent. Together with the automatic conversion, this means that code that depends on the long/int distinction is fragile – Python 2 didn’t provide very strong guarantees regarding the distinction.

If your code relies on the distinction, you will need to modify it.

Once your code does not rely on the long/int distinction, you can replace all calls to long with int. The recommended fixer will do this.

The L suffix not allowed in numeric literals

  • Fixer: python-modernize -wnf lib2to3.fixes.fix_numliterals (but see below)
  • Prevalence: Very common

In Python 2, 12345L designated a long literal. For numbers that exceed the range of int, the L suffix was optional: 1234567890123456789012345678901234567890 always named a long on current architectures.

In Python 3, the L suffix is not allowed.

In code that does not depend on the int/long distinction, you can simply drop the L suffix. The recommended fixer will do this, along with octal literal fixes described below.

If the specific type is important, you will need to refactor the code so that it does not rely on the distinction, as discussed above.

The L suffix dropped from the representation

  • Fixer: None
  • Prevalence: Rare

In Python 2, canonical representations of long integers included the L suffix. For example, repr(2**64) was 18446744073709551616L on most systems. In Python 3, the suffix does not appear. Note that this only affected repr, the string representation (given by str() or print()) had no suffix.

The canonical representations are rarely used, except in doctests.

As discussed previously, relying on the int/long distinction is fragile. By extension, relying on the output of repr of long numbers is also fragile. Call str() instead of repr() when the result might be a (long) integer.

Octal Literals

  • Fixer: python-modernize -wnf lib2to3.fixes.fix_numliterals (but see below)
  • Prevalence: Uncommon

Python 2’s other holdover from C-based languages is the syntax of octal literals: zero-prefixed numbers are interpreted in base 8. For example, the value of 0123 was 83, and 0987 caused a rather unhelpful SyntaxError. This is surprising to those not familiar with C, and it can lead to hard-to-spot errors.

Python 2.6 introduced the 0o prefix as an alternative to plain 0. Python 3 drops the 0 prefix: integer literals that start with 0 are illegal (except zero itself, and 0x/0o/0b prefixes).

You will need to change the leading zero in all 0-prefixed literals to 0o. The recommended fixer will do this automatically, along with long literal fixes described above.