Python 3.8 new features: the walrus operator, positional-only parameters, and much more

Earlier this month, the team behind Python announced the release of Python 3.8b2, the second of four planned beta releases. Ahead of the third beta release, which is scheduled for 29th July, we look at some of the key features coming to Python 3.8.

The "incredibly controversial" walrus operator

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 were excited as the operator does make the code a tiny bit more readable.

At the end of the PEP discussion, Guido van Rossum stepped down as BDFL (benevolent dictator for life) and the creation of a new governance model. In an interview with InfoWorld, Guido shared, “The straw that broke the camel’s back was a very contentious Python enhancement proposal, where after I had accepted it, people went to social media like Twitter and said things that really hurt me personally. And some of the people who said hurtful things were actually core Python developers, so I felt that I didn’t quite have the trust of the Python core developer team anymore.”

According to PEP 572, the assignment expression is a syntactical operator that allows you to assign values to a variable as a part of an expression. Its aim is to simplify things like multiple-pattern matches and the so-called loop and a half. At PyCon 2019, Dustin Ingram, a PyPI maintainer, gave a few examples where you can use this syntax:

Balancing lines of codes and complexity

Avoiding inefficient comprehensions

Avoiding unnecessary variables in scope

You can watch the full talk on YouTube:

https://www.youtube.com/watch?v=6uAvHOKofws

The feature was implemented by Emily Morehouse, Python core developer and Founder, Director of Engineering at Cuttlesoft, and was merged earlier this year:

https://twitter.com/emilyemorehouse/status/1088593522142339072

Explaining other improvements this feature brings, Jake Edge, a contributor on LWN.net wrote, “These and other uses (e.g. in list and dict comprehensions) help make the intent of the programmer clearer. It is a feature that many other languages have, but Python has, of course, gone without it for nearly 30 years at this point. In the end, it is actually a fairly small change for all of the uproars it caused.”

Positional-only parameters

Proposed in PEP 570, this introduces a new syntax (/) to specify positional-only parameters in Python function definitions. This is similar to how * indicates that the arguments to its right are keyword only. This syntax is already used by many CPython built-in and standard library functions, for instance, the pow() function:

pow(x, y, z=None, /)

This syntax gives library authors more control over better expressing the intended usage of an API and allows the API to “evolve in a safe, backward-compatible way.” It gives library authors the flexibility to change the name of positional-only parameters without breaking callers. Additionally, this also ensures consistency of the Python language with existing documentation and the behavior of various "builtin" and standard library functions.

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.

Debug support for f-strings

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. In Python 3.8, some additional syntax changes have been made by adding add (=) specifier and a !d conversion for ease of debugging.

You can use this feature like this:

print(f'{foo=} {bar=}')

This provides developers a better way of doing “print-style debugging”, especially for those who have a background in languages that already have such feature such as Perl, Ruby, JavaScript, etc. One developer expressed his 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.”

Python Initialization Configuration

Though Python is highly configurable, its configuration seems scattered all around the code. The PEP 587 introduces a new C API to configure the Python Initialization giving developers finer control over the configuration and better error reporting. Among the improvements, this API will bring include ability to read and modify configuration before it is applied and overriding how Python computes the module search paths (``sys.path``).

Along with these, there are many other exciting features coming to Python 3.8, which is currently scheduled for October, including a fast calling protocol for CPython, Vectorcall, support for out-of-band buffers in pickle protocol 5, and more.

You can find the full list on Python’s official website.

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