Bug 1673075

Description of problem:

The page https://dnf.readthedocs.io/en/latest/api_queries.html describes now query() might be used. The first example shows:

q = base.sack.query()
i = q.installed()
i = i.filter(name='pepper')
packages = list(i) # i only gets evaluated here

a = q.available()
a = a.filter(name='pepper')
for pkg in a: # a only gets evaluated here
    print(pkg.name)

and the first part can be made working in standalone script as

#!/usr/bin/python3
import dnf
base = dnf.Base()
base.fill_sack()
q = base.sack.query()
i = q.installed()
for p in i:
        print(p, p.reponame)

I myself would have preferred to have such a standalone example shown but the q.installed() part works well.

However, the q.available() part does not work out of box -- running

#!/usr/bin/python3
import dnf
base = dnf.Base()
base.fill_sack()
q = base.sack.query()
a = q.available()
a = a.filter(name = "zsh")
for p in a:
        print(p, p.reponame)

does not print anything even if running

dnf list zsh

works fine.

It'd be nice to have complete examples, not just snippets without context and without obvious way of making them working.

Version-Release number of selected component (if applicable):

dnf-4.0.9-2.fc29.noarch

How reproducible:

Deterministic.

Steps to Reproduce:
1. Read https://dnf.readthedocs.io/en/latest/api_queries.html.
2. Be happy that there is available() which is documented to
    Return a new query limiting the original query to the not-installed packages, that is packages available from the repositories.
3. Based on the example snippet on that page, create and run script

#!/usr/bin/python3
import dnf
base = dnf.Base()
base.fill_sack()
q = base.sack.query()
a = q.available()
a = a.filter(name = "zsh")
for p in a:
        print(p, p.reponame)


Actual results:

Nothing printed.

Expected results:

The zsh package information printed.

Additional info:

Of course, it's well possible that the example and my approach is correct and the bug is in the dnf code -- in that case the code should be fixed. ;-)