Prepend line to beginning of a file

by

In modes 'a' or 'a+', any writing is done at the end of the file, even if at the current moment when the write() function is triggered the file’s pointer is not at the end of the file: the pointer is moved to the end of file before any writing. You can do what you want in two manners.

1st way, can be used if there are no issues to load the file into memory:

def line_prepender(filename, line):
    with open(filename, 'r+') as f:
        content = f.read()
        f.seek(0, 0)
        f.write(line.rstrip('\r\n') + '\n' + content)

2nd way:

def line_pre_adder(filename, line_to_prepend):
    f = fileinput.input(filename, inplace=1)
    for xline in f:
        if f.isfirstline():
            print line_to_prepend.rstrip('\r\n') + '\n' + xline,
        else:
            print xline,

I don’t know how this method works under the hood and if it can be employed on big big file. The argument 1 passed to input is what allows to rewrite a line in place; the following lines must be moved forwards or backwards in order that the inplace operation takes place, but I don’t know the mechanism

More Related Contents:

  1. How do I change the size of figures drawn with Matplotlib?
  2. Why can’t Python parse this JSON data? [closed]
  3. Send HTML emails with Python
  4. ‘Webdrivers’ executable may have wrong permissions. Please see https://sites.google.com/a/chromium.org/chromedriver/home
  5. Python list rotation [duplicate]
  6. How to reduce a jpeg size to a ‘desired size’?
  7. Why does assigning to my global variables not work in Python?
  8. Could not load dynamic library ‘cudart64_101.dll’ on tensorflow CPU-only installation
  9. What is the fastest way to parse large XML docs in Python?
  10. Best way to create a “reversed” list in Python?
  11. Is it possible to change sprite colours in Pygame?
  12. asyncio.sleep() vs time.sleep()
  13. why sum on lists is (sometimes) faster than itertools.chain?
  14. Using psycopg2 with Lambda to Update Redshift (Python)
  15. How to use __setattr__ correctly, avoiding infinite recursion
  16. Python and MySQLdb: substitution of table resulting in syntax error
  17. pyclipper installation error: “tp_print is not a member of _typeobject”
  18. How to update-alternatives to Python 3 without breaking apt?
  19. Repeat Rows in Data Frame n Times [duplicate]
  20. How to append multiple values to a list in Python
  21. str.translate gives TypeError – Translate takes one argument (2 given), worked in Python 2
  22. Why can’t I import from a module alias?
  23. Why isn’t my class initialized by “def __int__” or “def _init_”? Why do I get a “takes no arguments” TypeError, or an AttributeError?
  24. How to attach a Scrollbar to a Text widget?
  25. Getting spline equation from UnivariateSpline object
  26. Techniques for working with large Numpy arrays? [duplicate]
  27. 3-dimensional array in numpy
  28. kill process with python
  29. How to smooth a curve for a dataset
  30. Dropping Root Permissions In Python

Leave a Comment