Can you elaborate on what change happened, after what command, and why it's unexpected?

@FiloSottile hi, after go mod vendor, there are lots of changes inVendor.
Since i only want move from dep to module, all things about dep like Vendor should stay as they were.

go mod vendor is more effective than dep at removing unneded files from the vendor folder. However, if the go mod init step went well, there should be no other meaningful change to the vendor folder.

If there is and you believe it is a bug, we will need instructions to reproduce the issues, and details on what changes.

I reproduce it:

@FiloSottile hi, any input?

go mod init does not promise to exactly reproduce the configuration of dep. It roughs in a starting point, from which the result is computed using minimal version selection and the transitive go.mod files of your dependencies.

To my knowledge, dep makes no effort to ingest or interpret the dependencies declared in go.mod files, so if any dependency has such a file, module mode can produce a substantially different result from dep. So merely being different from dep is not an error.

If you believe that the module-mode result is erroneous, please provide specific detail for which module's (or modules') requirements are not being followed. (#29773 is the only such issue that I know of at the moment.)

@bcmills hi, my key issue is that why go mod modify things under vendor, go mod do not need them any more, right?

Hi, i do below test in clean env:

$ docker run golang:1.12.1 /bin/bash
# go get github.com/cortexproject/cortex
# cd /go/src/github.com/cortexproject/cortex
# export GO111MODULE=on
# go mod init
# go mod vendor

Then i can see lots of changes in vendor.
@bcmills i test 3 times, reproduce every time.

@bcmills hi, any update?

Your steps to reproduce explicitly invoke go mod vendor. The whole point of go mod vendor is to update the vendor directory.

As noted in #31223 (comment): if you believe that the module-mode result is erroneous, please provide specific detail for which module's (or modules') requirements are not being followed.

Get it, go mod vendor not only get things from vendor and also update it.

But it may be confused for users who want to move dep to go mod, since they would remove vendor, update vendor could be meaningless for them.

Add an option to avoid changes in vendor could be better i think.

go mod vendor does not “get things from vendor” at all: its only purpose is to update the vendor directory, and it currently begins that process by deleting it entirely (without reading from it) before repopulating it.

If you want to avoid changes in vendor, then simply don't run go mod vendor. (But see also #30240.)