CODEWITHSUNDEEP
pythonf-string
Flimm
Flimm

Reputation: 150643

Is there a way to include a comment in an f-string?

It would be useful for mo to include a comment in an f-string. For instance, take this code:

f"""
<a
   href="{ escape(url) }"
   target="_blank" { # users expect link to open in new tab }
>bla</a>
"""

It would be nice if this code was equivalent to:

f"""
<a
   href="{ escape(url) }"
   target="_blank" 
>bla</a>
"""

You can include full Python expressions in between the curly brackets, but it looks like you can't include comments. Am I correct? Is there a way to do this?

Upvotes: 5

Views: 2108

Answers (4)

Paul Donohue
Paul Donohue

Reputation: 1171

Since the question specifically asked about Python comments in f-strings, Asocia's answer is technically correct; You cannot include Python comments between curly brackets in f-strings.

However, if you are willing to abuse other Python features to emulate comments, then there are other ways to incorporate text into an f-string in source code such that it will not be included in the output of the f-string at run time.

For example:

f"""
<a
   href="{ escape(url) }"
   target="_blank"{ '# users expect link to open in new tab' :.0 }
>bla</a>
"""

Some options:

  • { 'my comment' :.0 } This works by putting the comment in a single-quoted Python string, then using a "format specifier" in the f-string expression to truncate this string to zero length. (":" is a special separator character in f-string expressions, and everything after it is interpreted as a format specifier. The ".0" format specifier sets a "precision" of zero which truncates the string to zero length.)
  • { '# my comment' :.0 } This is the same as the above option, but adding "#" at the beginning of the string may help make it more obvious that this is intended to be a comment.
  • { 'my comment' [:0] } This uses a Python "slice" to return a zero-length substring of the Python string, instead of using a "format specifier" to truncate the string.
  • { 'my comment' and '' } This works by using the and boolean expression to evaluate the first string then (since any non-empty string is evaluated as true) return the second (empty) string.
  • { 'my comment' if False else '' } This works by using a conditional expression (sometimes called a "ternary operator") to ignore the first string and return the second (empty) string.
  • { comment('my comment') } If you are worried about other developers not understanding how the above expressions work or not recognizing that they are intended to be comments, then this may be more explicit. This requires a comment function to be defined separately as shown below.

Example comment function definition:

def comment(_):
  return ''

Upvotes: 3

Serge Ballesta
Serge Ballesta

Reputation: 148900

You cannot write a comment inside an expression. But you can write a string in multiple fragments and write a comment between 2 fragments provided the next fragment starts on a different line:

s = (f"""
<a
   href="{ escape(url) }"
   target="_blank" """ # users expect link to open in new tab
f""">bla</a>
""")

Upvotes: 2

Aaron
Aaron

Reputation: 1368

No. There is no comment in f-string.

When building a str, template engines may be overkill. Joining a list of str may be desirable.

s = ''.join([
    '<a',
    f' href="{escape(url)}"',
    ' target="_blank">',
    # users expect link to open in new tab
    'bla</a>',
])

Upvotes: 1

sahinakkaya
sahinakkaya

Reputation: 6056

From PEP498:

Comments, using the '#' character, are not allowed inside an expression.

There is no way to comment other than putting a '#' character in Python, so it is not possible.

Upvotes: 8

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