# Porting to Python 3

## Introduction

This document covers Twisted-specific issues in porting your code to Python 3.

## API Differences

### twisted.python.failure

`Failure.trap` raises itself (i.e. a `Failure` ) in Python 2. In Python 3, the wrapped exception will be re-raised.

## Byte Strings and Text Strings

Several APIs which on Python 2 accepted or produced byte strings (instances of `str` , sometimes just called bytes ) have changed to accept or produce text strings (instances of `str` , sometimes just called text or unicode ) on Python 3.

From `twisted.internet.address` , the `IPv4Address` and `IPv6Address` classes have had two attributes change from byte strings to text strings: `type` and `host` .

`twisted.python.log` has shifted significantly towards text strings from byte strings. Logging events, particular those produced by a call like `msg("foo")` , must now be text strings. Consequently, on Python 3, event dictionaries passed to log observes will contain text strings where they previously contained byte strings.

`twisted.python.runtime.platformType` and the return value from `twisted.python.runtime.Platform.getType` are now both text strings.

`twisted.python.filepath.FilePath` has not changed. It supports only byte strings. This will probably require applications to update their usage of `FilePath` , at least to pass explicit byte string literals rather than “native” string literals (which are text on Python 3).

`reactor.addSystemEventTrigger` arguments that were previously byte strings are now native strings.

`twisted.names.dns` deals with strings with a wide range of meanings, often several for each DNS record type. Most of these strings have remained as byte strings, which will probably require application updates (for the reason given in the `FilePath` section above). Some strings have changed to text strings, though. Any string representing a human readable address (for example, `Record_A` ‘s `address` parameter) is now a text string. Additionally, time-to-live (ttl) values given as strings must now be given as text strings.

`twisted.web.resource.IResource` continues to deal with URLs and all URL-derived values as byte strings.

`twisted.web.resource.ErrorPage` has several string attributes (`template` , `brief` , and `detail` ) which were previously byte strings. On Python 3 only, these must now be text strings.