# Python F-Strings Are Fun!

In python 3.6 we saw the adoption of Literal String Interpolation, or as they are known more commonly, f-strings. At first I was hesitant because... well... we've got multiple string tools already available:

one, two = 1, 2
_format = '{},{}'.format(one, two)
_percent = '%s,%s' % (one, two)
_concatenation = str(one) + ',' + str(two)
_join = ','.join((str(one),str(two)))
assert _format == _percent == _concatenation == _join


Adding f-strings to this mix didn't seem all that useful:

_fstring = f'{one},{two}'
assert _fstring == _format == _percent == _concatenation == _join


I was doubtful, but then I tried out f-strings on a non-trivial example. Now I'm hooked. Be it on local utility scripts or production code, I now instinctively gravitate toward their usage. In fact, f-strings are so useful that going back to earlier versions of Python now feels cumbersome.

The reason why I feel this way is that f-strings are concise but easy to understand. Thanks to intuitive expression evaluation I can compress more verbose commands into smaller lines of code that are more legible. Take a look:

_fstring = f'Total: {one + two}'  # Go f-string!
_format = 'Total: {}'.format(one + two)
_percent = 'Total: %s' % (one + two)
_concatenation = 'Total: ' + str(one + two)
assert _fstring == _format == _percent == _concatenation


The f-string example is four characters shorter than the closest alternative and is extremely easy to read. Indeed, put the f-string example in front of a non-programmer and they'll understand it fast. The same won't apply to the alternatives, odds are they'll ask what .format(), str(), and the % mean.

The conciseness and power of the intuitive expression evaluation can't be understated. On the surface f-strings seem like a small step forward for Python, but once I started using them I realized they were a huge step in codability for the language.

Now I'm hooked. I'm addicted to f-strings. When I step back to Python 3.5 or lower I feel like less of a Python coder. Yes, I have a problem with how much I lean on f-strings now, but I acknowledge my problem. I would go to therapy for it, but I believe I can manage the addiction for now.

Okay, enough joking, f-strings are awesome. Try them out.

# A Utility Script Example

We just released Two Scoops of Django 1.11, which is written in LaTeX. Like most programming books we provide code examples in a repo for our readers. However, as we completey revised the code-highlighting, we had to rewrite our code extractor from the ground up. In a flurry of cowboy coding, I did so in thirty minutes using Python 3.6 while leaning on f-strings:

"""Two Scoops of Django 1.11 Code Extractor"""
import os
import shutil
from glob import glob

try:
shutil.rmtree('code')
print('Removed old code directory')
except FileNotFoundError:
pass
os.mkdir('code')
print('Created new code directory')

STAR = '*'

LEGALESE = """LEGAL TEXT GOES HERE"""

LANGUAGE_START = {
'\\begin{python}': '.py',
'\\begin{django}': '.html',
'\\begin{plaintext}': '.txt',
'\\begin{sql}': '.sql',
'\\begin{makefile}': '',
'\\begin{json}': '.json',
'\\begin{bash}': '.txt',
'\\begin{xml}': '.html',
}

LANGUAGE_END = {x.replace('begin', 'end'):y for x,y in LANGUAGE_START.items()}

def is_example(line, SWITCH):
for key in SWITCH:
if line.strip().startswith(key):
return SWITCH[key]
return None

def makefilename(chapter_num, in_example):
return f'code/chapter_{chapter_num}_example_{str(example_num).zfill(2)}{in_example}'

if __name__ == '__main__':

in_example = False
starting = False
for path in glob('chapters/*.tex'):
try:
chapter_num = int(path[9:11])
chapter_num = path[9:11]
except ValueError:
if not path.lower().startswith('appendix'):
print(f'{STAR*40}\n{path}\n{STAR*40}')
continue
example_num = 1
with open(path) as f:
lines = (x for x in f.readlines())
for line in lines:
if starting:
# Crazy long string interpolation that should probably
# be broken up but remains because it's easy for me to read
filename =  f'code/chapter_{chapter_num}_example_{str(example_num).zfill(2)}{in_example}'
dafile = open(filename, 'w')
if in_example in ('.py', '.html'):
dafile.write(f'"""\n{LEGALESE}"""\n\n')
else:
dafile.write(f'{LEGALESE}\n{STAR*20}\n\n')
print(filename)
if not in_example:
mime = None
in_example = is_example(line, LANGUAGE_START)
if in_example:
starting = True
continue
mime = is_example(line, LANGUAGE_END)
starting = False
if mime:
print(mime)
in_example = False
example_num += 1
dafile.close()
else:
dafile.write(line)

Tags: twoscoops python django python python3