How can I remove a trailing newline?

Question

Try the method rstrip() (see doc Python 2 and Python 3)

>>> 'test string\n'.rstrip()
'test string'

Python's rstrip() method strips all kinds of trailing whitespace by default, not just one newline as Perl does with chomp.

>>> 'test string \n \r\n\n\r \n\n'.rstrip()
'test string'

To strip only newlines:

>>> 'test string \n \r\n\n\r \n\n'.rstrip('\n')
'test string \n \r\n\n\r '

In addition to rstrip(), there are also the methods strip() and lstrip(). Here is an example with the three of them:

>>> s = "   \n\r\n  \n  abc   def \n\r\n  \n  "
>>> s.strip()
'abc   def'
>>> s.lstrip()
'abc   def \n\r\n  \n  '
>>> s.rstrip()
'   \n\r\n  \n  abc   def'

And I would say the "pythonic" way to get lines without trailing newline characters is splitlines().

>>> text = "line 1\nline 2\r\nline 3\nline 4"
>>> text.splitlines()
['line 1', 'line 2', 'line 3', 'line 4']

The canonical way to strip end-of-line (EOL) characters is to use the string rstrip() method removing any trailing \r or \n. Here are examples for Mac, Windows, and Unix EOL characters.

>>> 'Mac EOL\r'.rstrip('\r\n')
'Mac EOL'
>>> 'Windows EOL\r\n'.rstrip('\r\n')
'Windows EOL'
>>> 'Unix EOL\n'.rstrip('\r\n')
'Unix EOL'

Using '\r\n' as the parameter to rstrip means that it will strip out any trailing combination of '\r' or '\n'. That's why it works in all three cases above.

This nuance matters in rare cases. For example, I once had to process a text file which contained an HL7 message. The HL7 standard requires a trailing '\r' as its EOL character. The Windows machine on which I was using this message had appended its own '\r\n' EOL character. Therefore, the end of each line looked like '\r\r\n'. Using rstrip('\r\n') would have taken off the entire '\r\r\n' which is not what I wanted. In that case, I simply sliced off the last two characters instead.

Note that unlike Perl's chomp function, this will strip all specified characters at the end of the string, not just one:

>>> "Hello\n\n\n".rstrip("\n")
"Hello"

Note that rstrip doesn't act exactly like Perl's chomp() because it doesn't modify the string. That is, in Perl:

$x="a\n";

chomp $x

results in $x being "a".

but in Python:

x="a\n"

x.rstrip()

will mean that the value of x is still "a\n". Even x=x.rstrip() doesn't always give the same result, as it strips all whitespace from the end of the string, not just one newline at most.