Syntax Changes

Python 3 cleaned up some warts of the language’s syntax.

The changes needed to accommodate this are mostly mechanical, with little chance of breaking code, so they work well as the first patches to send to a project when intending to port it.

Tabs and Spaces

  • Fixer: see below
  • Prevalence: Very common (unless the code uses a style linter)

In Python 2, a tab character in indentation was considered equal to 8 spaces or less. In Python 3, a tab is only equal to another tab, so the following code is rejected (whitespace highlighted):

def f(cond):
····if cond:
→       do_something()
→       do_something_else()

If your code mixes tabs and spaces, the easiest way to fix this is converting all tabs to spaces. You can use the following Bash command for this:

find . -name '*.py' -type f -exec bash -c 'T=$(mktemp); expand -i -t 8 "$0" > "$T" && mv "$T" "$0"' {} \;

Tuple Unpacking in Parameter Lists

  • Fixer: python-modernize -wnf lib2to3.fixes.fix_tuple_params (fixup needed)
  • Prevalence: Common

Python 3 requires that each argument of a def function has a name. This simplifies code that uses introspection (such as help systems, documentation generation, and automatic dispatchers), but it does have a drawback: tuples are no longer allowed in formal parameter lists.

For example, functions like these are no longer allowed in Python 3:

def line((x1, y1), (x2, y2)):
    connect_points(Point(x1, y1), Point(x2, y2))

lambda (key, item): (item, key)

The recommended fixer does a good job in finding places that need fixing, but it does need some manual cleanup. The above example would be rewritten to:

def line(xxx_todo_changeme, xxx_todo_changeme1):
    (x1, y1) = xxx_todo_changeme
    (x2, y2) = xxx_todo_changeme1
    connect_points(Point(x1, y1), Point(x2, y2))

lambda key_item: (key_item[1], key_item[0])

For def, each of the newly introduced variables should be renamed to something more appropriate.

As for lambda, this transformation can leave the code less readable than before. For each such lambda, you should consider if replacing it with a regular named function would be an improvement.


  • Fixer: python-modernize -wnf lib2to3.fixes.fix_repr (with caveat)
  • Prevalence: Common

The backtick (`) operator was removed in Python 3. It is confusingly similar to a single quote, and hard to type on some keyboards. Instead of the backtick, use the equivalent built-in function repr().

The recommended fixer does a good job, though it doesn’t catch the case where the name repr is redefined, as in:

repr = None

which becomes:

repr = None

Re-defining built-in functions is usually considered bad style, but it never hurts to check if the code does it.

The Inequality Operator

  • Fixer: python-modernize -wnf lib2to3.fixes.fix_ne
  • Prevalence: Rare

In the spirit of “There’s only one way to do it”, Python 3 removes the little-known alternate spelling for inequality: the <> operator.

The recommended fixer will replace all occurrences with !=.

New Reserved Words

  • Fixer: None
  • Prevalence: Rare

In Python 3, None, True and False are syntactically keywords, not variable names, and cannot be assigned to. This was partially the case with None even in Python 2.6.

Hopefully, production code does not assign to True or False. If yours does, figure a way to do it differently.

Other Syntax Changes

For convenience and completeness, this section lists syntax changes covered in other chapters: