U Modifier in Python File Opening

When the U modifier is used while opening a file then Python opens the file in Universal Newline mode. This mode enables Python to automatically detect and handle all common newline characters including \n, \r\n and \r during file reading. It is particularly useful when working with text files generated on various operating systems such as Windows, macOS or Linux which use different newline conventions.

The U mode was used in Python 2 and early Python 3 versions to enable newline normalization. It allowed consistent handling of line endings regardless of the platform on which the file was created. However, this mode has been deprecated from Python 3.4 as it is no longer necessary, as universal newline mode is now the default behavior when reading files in text mode r.

Behavior of U Modifier in Earlier Python Versions

In earlier versions of Python, using the U modifier was common when reading files containing mixed newline characters. It allowed seamless processing without worrying about inconsistent line breaks.

Following is an example that shows reading a file with various newline characters using the U mode -

with open("example.txt", "rU") as file:
   lines = file.readlines()
   for line in lines:
      print(repr(line))

Below is the output from a file containing mixed newlines -

'Line 1\n'
'Line 2\n'
'Line 3\n'

Using U Modifier in the current versions

In modern versions of Python 3.4 and above, the universal newline support is included automatically when opening files in text mode. This means we don't need to use the U modifier anymore, and attempting to use it will raise a ValueError.

# Modern approach (Python 3.4+)
with open("example.txt", "r") as file:
   lines = file.readlines()
   for line in lines:
      print(repr(line))

Here is the output of the above program -

'Line 1\n'
'Line 2\n'
'Line 3\n'