Python 3.8 is now available with walrus operator, positional-only parameters support for Vectorcall, and more

Yesterday, the latest version of the Python programming language, Python 3.8 was made available with multiple new improvements and features. Features include the new walrus operator and positional-only parameters, runtime audit Hooks, Vectorcall, a fast calling protocol for CPython and more. Earlier this month, the team behind Python announced the release of Python 3.8b2, the second of four planned beta releases.

What’s new in Python 3.8

PEP 572: New walrus operator in assignment expressions

Python 3.8 has a new walrus operator := that assigns values to variables as part of a larger expression. It is useful when matching regular expressions where match objects are needed twice. It can also be used with while-loops that compute a value to test loop termination and then need that same value again in the body of the loop. It can also be used in list comprehensions where a value computed in a filtering condition is also needed in the expression body.

The walrus operator was proposed in PEP 572 (Assignment Expressions) by Chris Angelico, Tim Peters, and Guido van Rossum last year. Since then it has been heavily discussed in the Python community with many questioning whether it is a needed improvement. Others are excited as the operator does make the code more readable.

One user commented on HN, “The "walrus operator" will occasionally be useful, but I doubt I will find many effective uses for it. Same with the forced positional/keyword arguments and the "self-documenting" f-string expressions. Even when they have a use, it's usually just to save one line of code or a few extra characters.”

https://twitter.com/reynoldsnlp/status/1183498971425042433

https://twitter.com/jakevdp/status/1140071525870997504

PEP 570: New function parameter syntax in positional-only parameters

Python 3.8 has a new function parameter syntax / to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments. This notation allows pure Python functions to fully emulate behaviors of existing C-coded functions. It can be used to preclude keyword arguments when the parameter name is not helpful. It also allows the parameter name to be changed in the future without the risk of breaking client code.

As with PEP 572, this proposal also got mixed reactions from Python developers. In support, one developer said, “Position-only parameters already exist in cpython builtins like range and min. Making their support at the language level would make their existence less confusing and documented.”

While others think that this will allow authors to “dictate” how their methods could be used. “Not the biggest fan of this one because it allows library authors to overly dictate how their functions can be used, as in, mark an argument as positional merely because they want to. But cool all the same,” a Redditor commented.

PEP 578: Python Audit Hooks and Verified Open Hook

Python 3.8 now has an Audit Hook and Verified Open Hook. These hooks allow applications and frameworks written in pure Python code to take advantage of extra notifications. They also allow embedders or system administrators to deploy builds of Python where auditing is always enabled. These are available from Python and native code.

PEP 587: New C API to configure the Python Initialization

Though Python is highly configurable, its configuration seems scattered all around the code. Python 3.8 adds a new C API to configure the Python Initialization providing finer control on the whole configuration and better error reporting. This PEP also adds _PyRuntimeState.preconfig (PyPreConfig type) and PyInterpreterState.config (PyConfig type) fields to internal structures. PyInterpreterState.config becomes the new reference configuration, replacing global configuration variables and other private variables.

PEP 590: Provisional support for Vectorcall, a fast calling protocol for CPython

A currently provisional Vectorcall protocol is added to the Python/C API. It is meant to formalize existing optimizations that were already done for various classes. Any extension type implementing a callable can use this protocol. It will be made fully public in Python 3.9.

PEP 574: Pickle protocol 5 supports out-of-band data buffers

The pickle protocol 5 now introduces support for out-of-band buffers. This means PEP 3118 compatible data can be transmitted separately from the main pickle stream, at the discretion of the communication layer.

Parallel filesystem cache for compiled bytecode files

There is a new PYTHONPYCACHEPREFIX setting that configures the implicit bytecode cache to use a separate parallel filesystem tree, rather than the default __pycache__ subdirectories within each source directory.

Python uses the same ABI whether it’s built-in release or debug mode

With Python 3.8, Python uses the same ABI whether it’s built-in release or debug mode. On Unix, when Python is built in debug mode, it is now possible to load C extensions built-in release mode and C extensions built using the stable ABI. On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. Also, on Unix, when Python is built in debug mode, import now also looks for C extensions compiled in release mode and for C extensions compiled with the stable ABI.

f-strings now have a = specifier

Formatted strings (f-strings) were introduced in Python 3.6 with PEP 498. It enables you to evaluate an expression as part of the string along with inserting the result of function calls and so on. Python 3.8 adds a = specifier to f-strings for self-documenting expressions and debugging. An f-string such as f'{expr=}' will expand to the text of the expression, an equal sign, then the representation of the evaluated expression.

One developer expressed their delight on Hacker News, “F strings are pretty awesome. I’m coming from JavaScript and partially java background. JavaScript’s String concatenation can become too complex and I have difficulty with large strings.”

Another developer said, “The expansion of f-strings is a welcome addition. The more I use them, the happier I am that they exist.” Someone added to this, “This makes clean string interpolation so much easier to do, especially for print statements. It's almost hard to use python < 3.6 now because of them.