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hilarious = False

joke_evaluation = "Isn't that joke so funny?! {}"

print(joke_evaluation.format(hilarious))

For the following lines of Python code I'm failing to understand a key concept.

A string is assigned to the variable joke_evaluationand include {} to embed another variable within it.

The third line of code has got me stuck, we are saying print the variable joke_evaluation then using the .format() function and passing another variable to it - hilarious which is set as a boolean data type.

Are the {} effectively functioning as placeholders? How does the .format() function know to populate the {} with the variable hilarious?

Please explain in basic terms if possible to increase my understanding, I'm failing to understand how Python populates the curly braces {} as I've mentioned above.

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  • yes, it's placeholders. and format is bound to str not to print Commented Jan 10, 2019 at 10:19

3 Answers 3

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Here's my understanding of the the format method:

Any string with curly braces {} will be replaced with the variable you have provided. So, if I have a string say:

myStr = "hello {}"

then doing: 

res = myStr.format("user")
print(res) #prints "hello user" without quotes.

Now, doing this:

res = myStr.format(123123)
print(res) #prints "hello 123123" without quotes.

As you might have guessed, the integer 123123 was implicitly converted to string before being included in the string.

Now, coming to the curly {} part:

  1. You can have multiple curly braces and must have the same number of parameters passed to format method. Eg: myStr = "hello {},{},{}, nice meeting you" res = myStr.format("abcd",123,"lol") print(res) #prints "hello abcd,123,lol, nice meeting you"
  2. You can even put indices in the {} to indicate position like {0} and {1}.
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1

Yes, {} acting as placeholders, which is treated by .format method in special way.

How does the .format() function know to populate the {} with the variable hilarious?

If you're providing {} only, it's substituted position-wise, i.e.

>>> 'first: {}, second: {}'.format(1, 2)
'first: 1, second: 2'

For more verbose or re-usable substitution you can use named arguments:

>>> "{actor1} tells {actor2} that he's {actor1}".format(actor1='Bob', actor2='Joel')
"Bob tells Joel that he's Bob"

More on awesome string formatting: pyformat.info

A bit more on formatting, when .format substituting placeholders with some objects, it calls __format__ method on it, which

  1. Accepts formatting spec — which gives you ability to control how it will be converted (for example, '{:.2f}'.format(3.1415)
  2. Return str, which will actually substitute placeholders
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you're forgetting that arguments are converted to str, which explains why booleans are printed as strings.
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Read Python documentation about string: https://docs.python.org/3.6/library/string.html?highlight=formatting

:) Everything you need to know. You can also change Python versions and see the behavior of formatting.

Scroll down to see examples and explanations.

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2 Comments

Just to expand I found this useful link that explains quite well with the Returntype etc: geeksforgeeks.org/python-format-function Is this kind of documentation that explains not available on the official Python docs?
@user1554264 it is! Just read the link above, it is all in there with examples and explanation.

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