It would be handy to be able to pass the `length` default argument defined by `shutil.copyfileobj()` to other public functions througout the module. When copying very large files (1GB +), increasing the memory buffer can divide the copy time in half and currently the only way to accomplish this is by overriding the needed function(s) which call `shutil.copyfileobj()`.

I propose a simple non-invasive change where the following functions will also expose the `length` kwarg and pass it downwards to `copyfileobj`:
- copyfile
- copy
- copy2

This seems reasonable to me.

How about increasing default value to 32KiB or 64KiB too?

binutils's cp works well for most cases, while it doesn't have option to specify blocksize.
I want Python's copy functions works nice enough for common cases too.

binutils cp uses 128KiB block for normal file. see more information in https://github.com/coreutils/coreutils/blob/master/src/ioblksize.h

But copyfileobj can be used other file-likes.
On Linux, typical socket buffer size is 128KiB, and typical pipe buffer size is 64KiB.  So block size larger than 64KiB may cause unneeded blocking.
That's why I suggested 32KiB or 64KiB.

Could you please provide any benchmarks Tyler?

bpo-33671 fixed this.