python File Input&Output

python

File IO

  • mode can be set as the Second parameter of openbuilt-in function
    default value is read only(r)
  1. r - read only - file object is created in read only, file pointer moves to the beginning of the file, Error occurs if the file doesn’t exists
  2. w - write only - creating new file or creating new file object in write only after removing original file’s content, file pointer moves to the beginning of the file
  3. a - addding content at the bottom - creating file object in write only or creating new file if the file doesn’t exist and then putting the file pointer to the end of the file

Binary file mode

  • rb
  • wb
  • ab

It is recommended to close the file object by close() immediately like f.close()

Using read() assigns whole content after reading everything

Reading file line by line

  1. Using forloof statement

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    f = open('t.txt')
    i = 1
    for line in f:
      print i, ":", line,
      i += 1
    f.close()

  2. readline() - from present file pointer to new line character = A line

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    f = open('t.txt')
    line = f.readline()
    i = 1
    while line:
      print i, ":", line,
      line = f.readline()
      i += 1
    f.close()

  3. readlines() - Saving line by line in a list, memory is used inefficiently
    It works differently depending on each cases - for instatement: not reading everything, taking line by line

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    f = open('t.txt')
    print f.readlines() # returning a list

    # file pointer moves to the beginning of the file
    f.seek(0)

    i = 1
    for line in f.readlines():
      print i, ":", line,
      i += 1
    f.close()

  4. xreadlines() - memory inefficiency improved

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    f = open('t.txt')
    print f.xreadlines() # returning file object

    # file pointer moves to the beginning of the file
    f.seek(0)

    i = 1
    for line in f.xreadlines():
      print i, ":", line,
      i += 1
    f.close()

for in or xreadlines() are recommended

the way to write down line by line

  1. writelines() - for writing line by line

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    lines = ['1st line\n', '2nd line\n', '3rd line\n']
    f = open('t1.txt','w')
    f.writelines(lines)
    f.close()

    f = open('t1.txt')
    print f.read() # reading whole content
    f.close()

  2. write() - putting \n between items

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    lines = ['1st line', '2nd line', '3rd line']
    f = open('t1.txt','w')
    f.write('\n'.join(lines))
    f.close()

    f = open('t1.txt')
    print f.read() # reading whole content
    f.close()

Adding content in the existing file existing

  • Using mode a
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    f = open('removeme.txt', 'a')
    f.write('3rd line\n')
    f.close()

    f = open('removeme.txt')
    print f.read()
    f.close()

Seeking specific file pointer

  • Sequential access
  • specific access
    • seek(n) - moving file pointer to the point which is n byte far from the beginning of the file
    • tell() - returning present file pointer in the file
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name = 't.txt'
f = open(name, 'w+') # read and write mode
s = '0123456789asdf'
f.write(s)

f.seek(5)
print f.tell()
print f.read(1)
print f.tell()

sys module’s standard IO(monitor) object

  • sys.stdout: standard Input and Output
  • sys.stderr: standard error Output
  • sys.stdin: standard Input

Saving a file by standard output

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import sys

f = open('t.txt','w')
stdout = sys.stdout # saving standard output
sys.stdout = f # changing standard output to file object
print 'Sample output'
pirnt 'Good'
print 'Good'
f.close()
sys.stdout = stdout # restoring standard output
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