Best of Python 3 f-strings

This piece is primarily meant for those new to Python. These include mathematicians, economists, and so on who want to use Python within a Jupyter environment. Here is a quick guide on how to make Best of Jupyter.

Quick Primer

If you are familiar with earlier Python versions, here are my top picks on how to move from .format () to this new one:

_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

f-string Magic

f-strings are how you should use print statements in Python. It is fairly reminiscent of LaTeX in it’s inline notation:

# inline variables, similar to LaTeX
name = "Fred"
print(f"He said his name is {name}.")
# 'He said his name is Fred.'

Notice how the variable name can now be used inline. This is a simple and easy to use syntax: just include the variable in surrounding {} while marking the string type as f-string using the ‘f’ in the beginning.

Note to the advanced programmer:

‘f’ may be combined with ‘r’ to produce raw f-string which can be used inside regex or similar functions. ‘f’ may not be combined with ‘u’, this is because all Python3.6+ strings are Unicode by default now. This means, you can write fstrings in Hindi, Chinese, French, Korean and atleast 10 other languages.

You can write fstrings in Hindi, Chinese, French, Korean and any language covered by Unicode.

But why are these called formatted-strings in the first place? Because you can use with some cool formatting hacks.

Simplified Alignment and Spacing

Have you ever tried creating a table such as that for logging or visualization? Arranging the elements becomes a nightmare with several \t tab characters flying around.

This is much easier with Python f-strings using the colon ‘:’ operator, followed by a an alignment operator and field width value.

There are atleast 3 alignment operator: < for left aligned, > for right aligned, and ^ for center aligned. Refer the code example:

correct = 'correct'
phonetic_correct = 'phonetic_correct'
typo = 'typo'
phonetic_typo = 'phonetic_typo'
phonetic_distance = 'phonetic_distance'
print(f'No Spacing:')
print(f'{correct}|{phonetic_correct}|{typo}|{phonetic_typo}|{phonetic_distance}|\n')
# No Spacing:
# correct|phonetic_correct|typo|phonetic_typo|phonetic_distance|
print(f'Right Aligned:')
print(f'{correct:>10}|{phonetic_correct:>20}|{typo:>10}|{phonetic_typo:>20}|{phonetic_distance:>20}|\n')
# Right Aligned:
#    correct|    phonetic_correct|      typo|       phonetic_typo|   phonetic_distance|
print(f'Left Aligned:')
print(f'{correct:<10}|{phonetic_correct:<20}|{typo:<10}|{phonetic_typo:<20}|{phonetic_distance:<20}|\n') 
# Left Aligned:
# correct   |phonetic_correct    |typo      |phonetic_typo       |phonetic_distance   |
print(f'Centre Aligned:')
print(f'{correct:^10}|{phonetic_correct:^20}|{typo:^10}|{phonetic_typo:^20}|{phonetic_distance:^20}|') 
# Centre Aligned:
#  correct  |  phonetic_correct  |   typo   |   phonetic_typo    | phonetic_distance  |

You also have support for decimal truncation and similar standard formatting utilities:

# auto-resolve variable scope when nested
width = 10
precision = 4
value = decimal.Decimal("12.34567")
print(f"result: {value:{width}.{precision}}")  # nested fields
# 'result:      12.35'

You might notice something interesting here: width and precision are automatically picked up from the scope. This means you can calculate width and precision using screen width or other inputs from system and use that.

Full Python Expressions Support

The above is only possible because the expression inside {} is actually being evaluated, or in programming terms: being executed.

This implies, that you can make any function call from within those {}.

Though, you should avoid doing this in practice very often because it might make your debugging very difficult. Instead, store the returned value from function in a variable and then add the variable in a fstring print statement.

Those coming from functional programming might miss their lambda functions. Don’t worry, Python has you covered:

Lambda Functions in f-strings 

# If you feel you must use lambdas, they may be used inside of parentheses:
print(f'{(lambda x: x*3)(3)}')
# '9'
# note that this returned a <str> and not <int>

Summary

  • f strings mean you can include variables and function calls inside your print statements
  • Inline variables: these are easier to read and debug for the developer
  • Use f-strings when you can!