Python 3.3: What's New, Release History & Support Lifecycle

What Is New in Python 3.3

Python 3.3 was released on September 29, 2012. The headline additions are the yield from syntax for delegating sub-generators (the foundation of what later became await), the Virtual Environments module venv, and a redesigned import system via importlib. Python 3.3 also introduced a more flexible string representation for Unicode strings, cutting memory usage significantly for ASCII-heavy workloads.

yield from -- Sub-Generator Delegation (PEP 380)

The yield from expression lets a generator delegate to a sub-generator transparently. Values from the inner generator flow out through the outer one, send() calls flow in, and the return value of the inner generator becomes the value of the yield from expression. This was the direct predecessor of await in Python 3.5.

def inner():
    yield 1
    yield 2
    return "inner done"

def outer():
    result = yield from inner()  # delegates to inner()
    print(f"inner returned: {result}")
    yield 3

list(outer())  # [1, 2, 3]; prints "inner returned: inner done"

Before yield from, delegating to a sub-generator required a manual for item in sub_gen: yield item loop -- which lost the ability to pass send() values and return values back to the caller.

venv -- Built-in Virtual Environments (PEP 405)

Python 3.3 ships venv as a standard module, replacing the third-party virtualenv for most use cases. A virtual environment is a lightweight, isolated Python installation with its own site-packages directory.

# Create a virtual environment
python3 -m venv .venv

# Activate (Linux/macOS)
source .venv/bin/activate

# Activate (Windows)
.venv\Scripts\activate.bat

# Install packages into the environment
pip install requests

The key difference from virtualenv: venv does not copy the full Python binary -- it uses symlinks (or shims on Windows), making environments smaller and faster to create.

Flexible String Representation (PEP 393)

Before Python 3.3, every Unicode string used UCS-2 or UCS-4 format regardless of content, meaning a string of 100 ASCII characters occupied 200 or 400 bytes. PEP 393 changed this so CPython uses the narrowest representation that fits the actual characters: Latin-1 for strings with codepoints up to U+00FF, UCS-2 up to U+FFFF, and UCS-4 only when needed. For typical English text, this cuts string memory by 3-4x and speeds up ASCII string operations.

Other Notable Additions in Python 3.3

• Namespace packages (PEP 420): A directory without an __init__.py file is now recognized as a "namespace package," allowing packages to be split across multiple directories or distribution packages.
• ipaddress module (PEP 3144): ipaddress.IPv4Address, IPv6Address, IPv4Network, IPv6Network -- clean objects for network programming without string parsing.
• faulthandler: Dumps a Python traceback on segfault or SIGFPE, which was previously impossible from within Python. Enable with python -X faulthandler or faulthandler.enable().
• Explicit exception chaining (PEP 3134): raise NewError from original_error sets the __cause__ attribute, and Python prints both exceptions and their chain when a traceback is displayed.
• lzma module: Python gains native LZMA/XZ compression -- the same format used for .tar.xz archives. Higher compression ratio than gzip or bzip2 at the cost of slower compression speed.