After a standard installation, I found this error in /var/log/zypp/history:

# 2017-09-07 15:43:23 python3-chardet-3.0.4-2.1.noarch.rpm installed ok
# Additional rpm output:
# /var/tmp/rpm-tmp.wMwH5i: line 2: update-alternatives: command not found
# warning: %post(python3-chardet-3.0.4-2.1.noarch) scriptlet failed, exit
status 127
# 
2017-09-07
15:43:23|install|python3-chardet|3.0.4-2.1|noarch||openSUSE-20170904-0|871c8546fa455d945d37149899ef47b6d8beed8d|

The problem is that python3-chardet is installed before update-alternatives and
at that time the binaries update-alternatives used in the postinstall
scriptlet, is not available yet.

A possible fix is to add this lines in python-chardet.spec:

 BuildRequires:  python-rpm-macros
+Requires(post): update-alternatives