Python Writing a 512-byte block of zeros to /dev/sda works, but doesn't behave like it is working

Question

Here I am trying to create a function to (eventually) support the wiping of drives using multiple specifications. The problem I am running in to, is when I try to write ZeroBlock to the disk, it gets written, but the code behaves as thou it has failed. the reason i say it worked is that it cleared the boot sector from the drive in my testing system

def WipeDisk(Drive, WipeSpec, Passes):
    DiskSize = int(System.HDD[Drive].Size)
    DiskSect = int(System.HDD[Drive].Sectors())
    SectSize = int(System.HDD[Drive].SectSz)
    System.HDD[Drive].Start = time.time()
    if (WipeSpec == "Zero"):
            with open("/dev/zero", "rb") as Zero:
                    ZeroBlock = Zero.read(SectSize)
            Zero.close()
            Pass = 0
            with open(System.HDD[Drive].Device, "wb") as Disk:
                    while (Pass < Passes):
                            Current = 1
                            while (Current < DiskSect):
                                    if (Disk.write(ZeroBlock)):
                                            if (Current %((DiskSect*Passes)/100) == 0):
                                                    (variable updates)
                                            if (Current == DiskSect):
                                                    Pass = (Pass+1)
                                    else:
                                            System.HDD[Drive].Error = 1
                                            Pass = Passes
                                            break
                                    Current = (Current+1)
                    if (Pass == Passes):
                            System.HDD[Drive].Current = Current
                            System.HDD[Drive].Percent = "100"
                            System.HDD[Drive].Complete = 1
                    Disk.close()
    else:
            print("Unknown Wipe Specification: "+WipeSpec)

the statements after if (Disk.write(ZeroBlock)): never executes. it jumps directly down to the associated else: where System.HDD[Drive].Error = 1 is set — AntonTakk, Mar 16 '15 at 19:51
There's a bunch of code there that doesn't seem necessary, while other stuff is missing. That said, your code is very un-Pythonic to start with, in particular you haven't grokked the idea of `with` yet. I'd clean all that up before even trying to find any errors in it, chances are you find and fix the errors on the way. — Ulrich Eckhardt, Mar 16 '15 at 20:35

score 1 · Accepted Answer · answered Mar 16 '15 at 19:54

1

The documentation for file.write says:

Write a string to the file. There is no return value.

You appear to be incorrectly assuming that write returns some value indicating whether the write succeeded. It doesn't.

answered Mar 16 '15 at 19:54

BrenBarn

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not sure how ive managed to miss that line every time ive looked at the manual for .write() That being the case, how would I go about determining the success or failure of a write()? for data desctuction, I can't simply hope it works, I need to verify – AntonTakk Mar 16 '15 at 20:06
@AntonTakk: It should raise an exception if there is some kind of I/O error. However, if you want to be extra sure for security purposes, you should read back the values from the disk and verify that they are zero. – BrenBarn Mar 16 '15 at 20:11

Python Writing a 512-byte block of zeros to /dev/sda works, but doesn't behave like it is working

1 Answers1