File object TRUNCATE

First, file.truncate does this:

Truncate the file’s size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position…

Not quite the same as Zed's description—it only "empties the file" if the current position is the start of the file—but since we just opened the file (and not in a mode), the current position is the start, so that isn't relevant. We're truncating to an empty file.
Which is all well and good, except that open already does that:

The most commonly-used values of mode are 'r' for reading, 'w' for writing (truncating the file if it already exists) …

So, we open the file, creating it if it doesn't exist and truncating it to 0 bytes if it does. Then, on the next line, we truncate it to 0 bytes.
(That "Truncating the file. Goodbye!" message is pretty misleading, since we've already truncated it. Imagine you put a breakpoint on that line and decided to kill the program before executing it…)
But notice that this isn't some silly mistake by Zed; he appears to have done this specifically to make the point in study drill #5:

If you open the file with 'w' mode, then do you really need the target.truncate()? Read the documentation for Python's open function and see if that's true.

copied form Stackflow answer.

Using truncate() we can access hidden and system files to erase data cause we cannot erase the hidden and system files' data using fp = open("file_name.txt","w")   fp.write("") like this.

It can be done like this.

Why we cannot do that:

"

It's just how the Win32 API works. Under the hood, Python's open function is calling the CreateFile function, and if that fails, it translates the Windows error code into a Python IOError.
The r+ open mode corresponds to a dwAccessMode of GENERIC_READ|GENERIC_WRITE and a dwCreationDisposition of OPEN_EXISTING. The w open mode corresponds to a dwAccessMode of GENERIC_WRITE and a dwCreationDisposition of CREATE_ALWAYS.
If you carefully read the remarks in the CreateFile documentation, it says this:

If CREATE_ALWAYS and FILE_ATTRIBUTE_NORMAL are specified, CreateFile fails and sets the last error to ERROR_ACCESS_DENIED if the file exists and has the FILE_ATTRIBUTE_HIDDEN or FILE_ATTRIBUTE_SYSTEM attribute. To avoid the error, specify the same attributes as the existing file.

So if you were calling CreateFile directly from C code, the solution would be to add in FILE_ATTRIBUTE_HIDDEN to the dwFlagsAndAttributes parameter (instead of just FILE_ATTRIBUTE_NORMAL). However, since there's no option in the Python API to tell it to pass in that flag, you'll just have to work around it by either using a different open mode or making the file non-hidden.

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copied from stackflow.