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File Guard and Large File Guard for preserving file content

Project description

File Guard and Large File Guard

support for preserving the old content of a file. support for large files provide access via seek() to a specific file position.

use in own code

use with context and rollback() to rewind the changes done.

when to use in own code

there are some typical use cases, such as:

  • in test scenarios when you need to rollback for the next test case
  • when wrting changed config files from your code (falls back to old state)
  • when randomly accessing larger files in binary mode

code for small files

for small files FileGuard makes a copy using a TemporaryFile. in error case or when calling rollback() the file is copied back.

import shutil
from fileguard import FileGuard, LargeFileGuard, dumpbim


def sample():

    # copy test file
    shutil.copy2("monty.bak.txt","monty.txt")

    try:
        with FileGuard("monty.txt",mode="+w",
                       always_restore=False,
                       blksize = 512,
                       debug=True,
                       ) as f:
            print("before write",f)
            f.write("content gone")
            f.flush()
            print("after write",f)

            raise Exception("something went wrong")

    except Exception as ex:
        print("err", ex)

    with open( "monty.txt", "rb" ) as f:
        c = f.read()
        print("---file content---")
        print(c)

code for large files

for large files LargeFileGuard provides methods like seek() and tell() to set the file pointer position. when writing to LargeFileGuard it preserves the content of the underlying file by writing to a journal or "before-image" file. in error case or by calling rollback() the changes are rewinded.

sample2():

    # copy test file
    shutil.copy2("monty.bak.txt","monty.txt")

    try:
        with LargeFileGuard("monty.txt","r+",
                                always_restore=False, # always rewind
                                keep_bim=True, # keep bim file, deleted otherwise
                                blksize=512,
                                debug=True,
                            ) as f:
            print("before write",f)
            f.write("!!! new content at 0 !!!")
            f.seek(100)
            f.write("!!! new content at 100 !!!")
            f.seek( 0, 2 )
            f.write("!!! new ending !!!")
            f.flush()
            print("after write",f)

            f.seek(0)
            c = f.read()
            print(c)

            # uncomment want to test...
            f.rollback()
            #raise Exception("something went wrong")

    except Exception as ex:

        print("err", ex)

    with open( "monty.txt", "rb" ) as f:
        c = f.read()
        print("---file content---")
        print(c)

    print( dumpbim("monty.txt") )

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