Python · basics

Comments

Notes for humans that Python skips: # comments and triple-quoted strings.

Single-line comments


          # this is a single line comment
          x = 5
          # ans: another comment
          y = 10

Everything after # on a line is ignored. The run shows which lines Python actually executes.

Inline comments


          # comment after code
          x = 5  # inline comment
          name = "Alice"  # another inline

The code before the # runs; the rest of the line is a note.

What Python sees

What you write


            # comment after code
            x = 5  # inline comment
            name = "Alice"  # another inline

What Python runs


            # comment after code
            x = 5  # inline comment
            name = "Alice"  # another inline

Multi-line comments


          """
          This is a
          multi-line comment
          or docstring
          """
          x = 5

Strictly, this is a string that is never stored anywhere, so it has no effect. At the top of a function it becomes the docstring.

Multi-line with single quotes


          '''
          This is also
          a multi-line comment
          '''
          y = 10

Three single quotes work exactly like three double quotes.

Comments don't execute


          # x = 10
          x = 5
          # ans: x is 5 (commented line ignored)

Putting # in front of a line "comments it out": a handy way to switch code off.

Exercises

Decide what runs and what doesn't, then press →.

Exercises · part 1


          # what is x?
          # ans: x = 10
          x = 5
          # ans: 5
          # what gets executed?
          x = 5  # x = 10
          # ans: x is 5

Exercises · part 2


          # multiline comment?
          """
          x = 10
          """
          x = 5
          # ans: x is 5

Exercises · part 3


          # can you do this?
          x = 5 """comment"""
          # ans: SyntaxError
          # what about this?
          x = 5  # comment
          y = x
          # ans: y is 5

Exercises · part 4


        # nested comments?
        # ans: # this is a comment
        # ans: valid, just a comment
        # multiline string as comment?
        '''
        This is also
        a comment
        '''
        # ans: valid if not assigned to anything

Exercises · part 5


          # what is x?
          x = 5
          # ans: x = 10
          # ans: x is 5
          # inline multiline?
          x = 5 """ this won't work """
          # ans: SyntaxError

Exercises · part 6


          # comment with backslash?
          x = 5  # this is \
          # ans: a comment
          # ans: two separate single-line comments

A backslash cannot continue a comment onto the next line; every comment line needs its own #.

Exercises · part 7


          # doc string vs comment?
          def func():
              """This is docstring"""
              # This is comment
              pass
          # ans: both valid, different purposes

The docstring is kept (try help(func)); the # comment is thrown away.

Remember

# to end of line

Everything after # is ignored, whether on its own line or after code.

Triple quotes

A string on its own is evaluated and thrown away, so it works as a multi-line comment.

Docstrings

A string first thing in a function documents it.

Not after code

x = 5 """text""" is a SyntaxError; only # can follow code.