Python string replace in a file without touching the file if no substitution was made

Question

What does Python\'s string.replace return if no string substitution was made? Does Python\'s file.open(f, \'w\') always touch the file even if no changes were made?

Answer 1

Your case is a particular case: 'newtext' has exactly the same number of characters as 'oldtext'.

Hence, it is possible to use one of the following codes to replace exactly a word 'oldtext' or a line in which the word 'oldtext' is present, by word 'newtext' or a line in which 'newtext' replaces 'oldtext'.

.

If files have not super-big size, the content of each file can be read entirely into memory:

from os import fsync      # code using find()

count = 0
for match in all_files('*.html', '.'):
    with open(match,'rb+') as thefile:
        diag = False
        fno = thefile.fileno()
        content = thefile.read()
        thefile.seek(0,0)
        x = content.find('oldtext')
        while x>=0:
            diag = True
            thefile.seek(x,1)
            thefile.write('newtext')
            thefile.flush()
            fsync(fno)
            x = content[thefile.tell():].find('oldtext')
    if diag:
        cnt += 1

or

from os import fsync     # code using a regex
import re
pat = re.compile('oldtext')

count = 0
for match in all_files('*.html', '.'):
    with open(match,'rb+') as thefile:
        diag = False
        fno = thefile.fileno()
        content = thefile.read()
        thefile.seek(0,0)
        prec = 0
        for mat in pat.finditer(content):
            diag = True
            thefile.seek(mat.start()-prec,1)
            thefile.write('newtext')
            thefile.flush()
            fsync(fno)
            prec = mat.end()
    if diag:
        cnt += 1

.

For heavy files, a reading and rewriting line after line is possible:

from os import fsync   # code for big files, using regex
import re
pat = re.compile('oldtext')

count = 0
for match in all_files('*.html', '.'):
    with open(match,'rb+') as thefile:
        diag = False
        fno = thefile.fileno()
        line = thefile.readline()
        while line:
            if 'oldtext' in line:
                diag = True
                thefile.seek(-len(line),1)
                thefile.write(pat.sub('newtext',line))
                thefile.flush()
                fsync(fno) 
            line = thefile.readline()
    if diag:
        cnt += 1

.

The instructions thefile.flush() and fsync(fno) are required after each writing in order that the file handler thefile points with accuracy on the exact position in the file at any moment. They allow to obtain effective writing ordered by instuction write()

flush() does not necessarily write the file’s data to disk. Use flush() followed by os.fsync() to ensure this behavior. http://docs.python.org/library/stdtypes.html#file.flush

.

These programs do the minimum. So I think they are fast.

.

Nota bene : a file opened in mode 'rb+' have no changing of its time of last modification if no modification has been performed.