F-Strings Are F'ing Cool Part 2

Posted on Sun 11 July 2021 in Posts

So in a previous post a couple years ago I wrote about some of the reasons the f-string functionality introduced in Python 3.6 was so useful. Today I learned a few new tricks with them from a recent episode of the Python Bytes Podcast so wanted to revisit the topic & share.


So how many times have you been debugging, wanted to know what the value of a variable was at a particular point, and then wrote something like:

>>> myvariable = 42
>>> print(f"The value of myvariable is: {myvariable}")
The value of myvariable is: 42

Ok, putting aside the fact I'm using print statements for debugging (😱), this is extremely common. But what if you had a second variable? Ok, so then you add it as well:

>>> myvariable = 42
>>> myothervariable = 99
>>> print(f"The value of myvariable is: {myvariable}  myothervariable: {myothervariable}")
The value of myvariable is: 42  myothervariable: 99

And so on. But I'm lazy, I don't want to repeat the variable name twice, so as of Python 3.8 there's a new specifier to aid with debugging. By appending an equal sign (=) to the expression the both the name & value get emitted:

>>> myvariable = 42
>>> myothervariable = 99
>>> print(f"The value of {myvariable=} {myothervariable=}")
The value of myvariable=42 myothervariable=99

Super handy. What's really crazy is it works with expressions too:

>>> myvariable = 42
>>> myothervariable = 99
>>> print(f"Calculation: {(myvariable * 3.14159 / myothervariable)=}")
Calculation: (myvariable * 3.14159 / myothervariable)=1.3327957575757574

This can be handy when printing out properties of an object:

>>> class Foo:
...     bar = 42
>>> f = Foo()
>>> print(f"{}")

It's also particularly handy if you have a function which takes arbitrary parameters via args and kwargs and you want to see what the values of them are:

>>> def foo(*args, **kwargs):
...     print(f"{args=} {kwargs=}")
>>> foo(242, 2342, "adlfk", arg1=9234, arg2="dfslkj")
args=(242, 2342, 'adlfk') kwargs={'arg1': 9234, 'arg2': 'dfslkj'}

Common Format Specifiers

So one of the things I talked about in my last post on F-strings was how you can use all the same format specifiers you used to use with .format() with F-strings. A common example of this is to format a float to a specific number of decimal places:

>>> import math
>>> print(f"Pi to 3 digits: {math.pi:.3f}")
Pi to 3 digits: 3.142

Or formatting datetimes:

>>> from datetime import datetime
>>> print(f"Today is { %d, %Y}")
Today is July 11, 2021

This is neat and all, but oftentimes these format strings are used in many places in a project. Say for example you want all dates in your project to be formatted like the example above. One approach would be to repeat that format string (%B %d, %Y) in each place you format a date. The problem though is if that format changes (say in the future you want 2 digit years, or want to include the day of the week, etc) you'd then have to search & replace every spot where you've used that format.

Ok, so this is software engineering 101, time for the "Don't Repeat Yourself" (or DRY) principle to apply, so maybe you extract out a function to format a datetime:

>>> def format_date(datetime_obj):
...     return f"{datetime_obj:%B %d, %Y}"
>>> print(f"Today is {format_date(}")
Today is July 11, 2021

Which works and is perfectly reasonable, but now you have this little function floating around which makes the f-string expression a little more cumbersome. One could also make a case that defining a function that is a single line of code is a bit overengineered (I'm not particularly sympathetic to this view myself, but some are). And really the format string is a constant, so arguably it's a little weird to have a function to express it's application.

But, here's where my mind was blown -- you can nest f-string expressions:

>>> print(f"Today is {{DATETIME_FORMAT}}")
Today is July 11, 2021

Now our format expression is a single constant variable, and we just defer to it wherever we want to format a datetime. If that format changes, we simply update the constant. Really useful trick.