Are you using this magic method for filesystems from Python 3.6?

Explore os.fspath and two other underutilized but still useful Python features.
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This is the seventh in a series of articles about features that first appeared in a version of Python 3.x. Python 3.6 was first released in 2016, and even though it has been out for a while, many of the features it introduced are underused and pretty cool. Here are three of them.

Separated numeral constants

Quick, which is bigger, 10000000 or 200000? Would you be able to answer correctly while scanning through code? Depending on local conventions, in prose writing, you would use 10,000,000 or 10.000.000 for the first number. The trouble is, Python uses commas and periods for other reasons.

Fortunately, since Python 3.6, you can use underscores to separate digits. This works both directly in code and when using the int() convertor from strings:

import math
math.log(10_000_000) / math.log(10)
    7.0
math.log(int("10_000_000")) / math.log(10)
    7.0

Tau is right

What's a 45-degree angle expressed in radians? One correct answer is π/4, but that's a little hard to remember. It's much easier to remember that a 45-degree angle is an eighth of a turn. As the Tau Manifesto explains, , called Τ, is a more natural constant.

In Python 3.6 and later, your math code can use the more intuitive constant:

print("Tan of an eighth turn should be 1, got", round(math.tan(math.tau/8), 2))
print("Cos of an sixth turn should be 1/2, got", round(math.cos(math.tau/6), 2))
print("Sin of a quarter turn should be 1, go", round(math.sin(math.tau/4), 2))
    Tan of an eighth turn should be 1, got 1.0
    Cos of an sixth turn should be 1/2, got 0.5
    Sin of a quarter turn should be 1, go 1.0

os.fspath

Starting in Python 3.6, there is a magic method that represents "convert to a filesystem path." When given an str or bytes, it returns the input.

For all types of objects, it looks for an __fspath__method and calls it. This allows passing around objects that are "filenames with metadata."

Normal functions like open() or stat will still be able to use them, as long as __fspath__ returns the right thing.

For example, here is a function that writes some data into a file and then checks its size. It also logs the file name to standard output for tracing purposes:

def write_and_test(filename):
    print("writing into", filename)
    with open(filename, "w") as fpout:
        fpout.write("hello")
    print("size of", filename, "is", os.path.getsize(filename))

You can call it the way you would expect, with a string for a filename:

write_and_test("plain.txt")
    writing into plain.txt
    size of plain.txt is 5

However, it is possible to define a new class that adds information to the string representation of filenames. This allows the logging to be more detailed, without changing the original function:

class DocumentedFileName:
    def __init__(self, fname, why):
        self.fname = fname
        self.why = why
    def __fspath__(self):
        return self.fname
    def __repr__(self):
        return f"DocumentedFileName(fname={self.fname!r}, why={self.why!r})"

Running the function with a DocumentedFileName instance as input allows the open and os.getsize functions to keep working while enhancing the logs:

write_and_test(DocumentedFileName("documented.txt", "because it's fun"))
    writing into DocumentedFileName(fname='documented.txt', why="because it's fun")
    size of DocumentedFileName(fname='documented.txt', why="because it's fun") is 5

Welcome to 2016

Python 3.6 was released about five years ago, but some of the features that first showed up in this release are cool—and underused. Add them to your toolkit if you haven't already.

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Moshe sitting down, head slightly to the side. His t-shirt has Guardians of the Galaxy silhoutes against a background of sound visualization bars.
Moshe has been involved in the Linux community since 1998, helping in Linux "installation parties". He has been programming Python since 1999, and has contributed to the core Python interpreter. Moshe has been a DevOps/SRE since before those terms existed, caring deeply about software reliability, build reproducibility and other such things.

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