Use A Python Format String In A Variable

July 2022

There are several ways to updates string variables with other variables. The two ways I use the most are .format() and Template Strings.

(Note that this is for dealing with strings in variables. If I'm outputting a string directly I use f-strings which are detailed in the notes further below)

I use the .format() option when there's not a lot of text (e.g. just one line) and the Template Strings when there's several lines (in which case I'm likely reading it in from a file.)

Using .format()

This is the basic setup. A string is stored in the initial_string variable with {} acting as a placeholder for where the replacement value will be inserted.

The next line use calls .format() with the single value we want to use to replace the {} placeholder.

printing it on the next line show our final `hello, world` output.

initial_string = "hello, {}"

  completed_string = initial_string.format('world')

  print(completed_string)

Output:

hello, world

You can use multiple {} placeholders as well. For example:

initial_string = 'the {} brown {}'

  completed_string = initial_string.format('quick', 'fox')

  print(completed_string)

Output:

the quick brown fox

Template Strings

I use template strings for longer texts. They use "named placeholders" that start with a $ (e.g. `$color`, `$fruit`, and $drink in this example)

Those values get filled in with their corresponding named arguments (e.g. `color='blue'`) when the template is filled out with `.substitute()`

For example:

from string import Template

  skeleton = Template("""
  Glue the sheet to the dark $color background.
  The juice of $fruit makes fine punch.
  A pot of $drink helps to pass the evening.
  """)

  completed_text = skeleton.substitute(
      color='blue',
      fruit='lemons',
      drink='tea'
  )

  print(completed_text)

Output:

Glue the sheet to the dark blue background.
  The juice of lemons makes fine punch.
  A pot of tea helps to pass the evening.

#+NOTES

This is what I use if I need to use a variable to hold the string that needs substitutions and replacements. Generally, I use that when I need to pass the string into a function, like:

from random import randint

  def add_random_number(template):
      update = template.format(randint(0,10))
      return update


  if __name__ == '__main__':
      template = 'The random number is: {}'
      output = add_random_number(template)
      print(output)

Output:

The random number is: 2

If I don't need to put the template for the string in a variable to pass it around I'll use an f-string directly. For example:

speed = 'quick'
  animal = 'fox'

  print(f'the {speed} brown {animal}')

Output:

the quick brown fox

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