From a developer’s point of view, the largest change in Python 3 is the handling of strings. In Python 2, the str type was used for two different kinds of values – text and bytes, whereas in Python 3, these are separate and incompatible types.

  • Text contains human-readable messages, represented as a sequence of Unicode codepoints. Usually, it does not contain unprintable control characters such as NULL.

    This type is available as str in Python 3, and unicode in Python 2.

    In code, we will refer to this type as unicode – a short, unambiguous name, although one that is not built-in in Python 3. Some projects refer to it as six.text_type (from the six library).

  • Bytes or bytestring is a binary serialization format suitable for storing data on disk or sending it over the wire. It is a sequence of integers between 0 and 255. Most data – images, sound, configuration info, or text – can be serialized (encoded) to bytes and deserialized (decoded) from bytes, using an appropriate protocol such as PNG, VAW, JSON or UTF-8.

    In both Python 2.6+ and 3, this type is available as bytes.

Ideally, every “stringy” value will explicitly and unambiguously be one of these types (or the native string, below). This means that you need to go through the entire codebase, and decide which value is what type. Unfortunately, this process generally cannot be automated.

We recommend replacing the word “string” in developer documentation (including docstrings and comments) with either “text”/“text string” or “bytes”/“byte string”, as appropriate.

The Native String

Additionally, code that supports both Python 2 and 3 in the same codebase can use what is conceptually a third type:

  • The native string (str) – text in Python 3, bytes in Python 2

Custom __str__ and __repr__ methods, and code that deals with Python language objects (such as attribute/function names) will always need to use the native string, because that is what each version of Python uses for internal text-like data. Developer-oriented texts, such as exception messages, could also be native strings.

For other data, you can use the native string in these circumstances:

  • You are working with textual data
  • Under Python 2, each “native string” value has a single well-defined encoding (e.g. UTF-8 or locale.getpreferredencoding())
  • You do not mix native strings with either bytes or text – always encode/decode diligently when converting to these types.

Native strings affect the semantics under Python 2 as little as possible, while not requiring the resulting Python 3 API to feel bad. But, having a third incompatible type makes the porting process harder. Native strings are suitable mostly for conservative projects, where ensuring stability under Python 2 justifies extra porting effort.

Conversion between text and bytes

It is possible to encode() text to binary data, or decode() bytes into a text string, using a particular encoding. By itself, a bytes object has no inherent encoding, so it is not possible to encode/decode without knowing the encoding.

It’s similar to images: an open image file might be encoded in PNG, JPG, or another image format, so it’s not possible to “just read” the file without either relying on external data (such as the filename), or effectively trying all alternatives. Unlike images, one bytestring can often be successfully decoded using more than one encoding.


Never assume a text encoding without consulting relevant documentation and/or researching a string’s use cases. If an encoding for a particular use case is determined, document it. For example, a function docstring can specify that some argument is “a bytestring holding UTF-8-encoded text data”, or module documentation may clarify that „as per RFC 4514, LDAP attribute names are encoded to UTF-8 for transmission“.

Some common text encodings are:

  • UTF-8: A widely used encoding that can encode any Unicode text, using one to four bytes per character.
  • UTF-16: Used in some APIs, most notably Windows and Java ones. Can also encode the entire Unicode character set, but uses two to four bytes per character.
  • ascii: A 7-bit (128-character) encoding, useful for some machine-readable identifiers such as hostnames (''), or textual representations of numbers ('1234', ''). Always check the relevant standard/protocol/documentation before assuming a string can only ever be pure ASCII.
  • locale.getpreferredencoding(): The “preferred encoding” for command-line arguments, environment variables, and terminal input/output.

If you are choosing an encoding to use – for example, your application defines a file format rather than using a format standardized externally – consider UTF-8. And whatever your choice is, explicitly document it.

Conversion to text

There is no built-in function that converts to text in both Python versions. The six library provides six.text_type, which is fine if it appears once or twice in uncomplicated code. For better readability, we recommend using unicode, which is unambiguous and clear, but it needs to be introduced with the following code at the beginning of a file:

if not six.PY2:
    unicode = str

Conversion to bytes

There is no good function that converts an arbitrary object to bytes, as this operation does not make sense on arbitrary objects. Depending on what you need, explicitly use a serialization function (e.g. pickle.dumps()), or convert to text and encode the text.

String Literals

Quoted string literals can be prefixed with b or u to get bytes or text, respectively. These prefixes work both in Python 2 (2.6+) and 3 (3.3+). Literals without these prefixes result in native strings.

Add a b or u prefix to all strings, unless a native string is desired.

String operations

In Python 3, text and bytes cannot be mixed. For example, these are all illegal:

b'one' + 'two'

b', '.join(['one', 'two'])

import re
pattern = re.compile(b'a+')

Type checking

  • Fixer: python-modernize -wnf libmodernize.fixes.fix_basestring
  • Prevalence: Rare

Because the str and unicode types in Python 2 could be used interchangeably, it sometimes didn’t matter which of the types a particular value had. For these cases, Python 2 provided the class basestring, from which both str and unicode derived:

if isinstance(value, basestring):
    print("It's stringy!")

In Python 3, the concept of basestring makes no sense: text is only represented by str.

For type-checking text strings in code compatible with both versions, the six library offers string_types, which is (basestring,) in Python 2 and (str,) in Python 3. The above code can be replaced by:

import six

if isinstance(value, six.string_types):
    print("It's stringy!")

The recommended fixer will import six and replace any uses of basestring by string_types.

File I/O

  • Fixer: python-modernize -wnf libmodernize.fixes.fix_open
  • Prevalence: Common

In Python 2, reading from a file opened by open() yielded the generic str. In Python 3, the type of file contents depends on the mode the file was opened with. By default, this is text strings; b in mode selects bytes:

with open('/etc/passwd') as f:  # text

with open('/bin/sh', 'rb') as f:  # bytes

On disk, all files are stored as bytes. For text-mode files, their content is decoded automatically. The default encoding is locale.getpreferredencoding(False), but this might not always be appropriate, and may cause different behavior across systems. If the encoding of a file is known, we recommend always specifying it:

with open('data.txt', encoding='utf-8') as f:

Similar considerations apply when writing to files.

The behavior of open is quite different between Python 2 and 3. However, from Python 2.6 on, the Python 3 version is available in the io module. We recommend replacing the built-in open function with, and using the new semantics – that is, text files contain unicode:

from io import open

with open('data.txt', encoding='utf-8') as f:

Note that under Python 2, the object returned by has a different type than that returned by open. If your code does strict type checking, consult the notes on the file() built-in.

The recommended fixer will add the from io import open import, but it will not add encoding arguments. We recommend adding them manually if the encoding is known.

Testing Strings

When everything is ported and tests are passing, it is a good idea to make sure your code handles strings correctly – even in unusual situations.

Many of the tests recommended below exercise behavior that “works” in Python 2 (does not raise an exception – but may produce subtly wrong results), while a Python 3 version will involve more thought and code.

You might discover mistakes in how the Python 2 version processes strings. In these cases, it might be a good idea to enable new tests for Python 3 only: if some bugs in edge cases survived so far, they can probably live until Python 2 is retired. Apply your own judgement.

Things to test follow.

Non-ASCII data

Ensure that your software works (or, if appropriate, fails cleanly) with non-ASCII input, especially input from end-users. Example characters to check are:

  • é ñ ü Đ ř ů Å ß ç ı İ (from European personal names)
  • ½ (alternate forms and ligatures)
  • ¥ (currency symbols)
  • Ж η (various scripts)
  • 🐍 💖 (symbols and emoji)

Encodings and locales

If your software handles multiple text encodings, or handles user-specified encodings, make sure this capability is well-tested.

Under Linux, run your software with the LC_ALL environment variable set to C and to tr_TR.utf8. Check handling of any command-line arguments and environment variables that may contain non-ASCII characters.

Invalid input

Test how the code handles invalid text input. If your software deals with files, try it a on non-UTF8 filename.

Using Python 3, such a file can be created by:

with open(b'bad-\xFF-filename', 'wb') as file: