Why pytest always says ” ImportError: attempted relative import with no known parent package”

I found I must add __init__.py in test and projroot. __init__.py in src is not necessary.
And pytest could run correctly in 3 folders:

# parent folder of project root
C:\>pytest projroot

# project root
C:\>cd projroot
C:\projroot>pytest

# test folder
C:\projroot>cd test
C:\projroot\test>pytest