Robot does not convert your test cases to python behind the scenes before running them. Instead, it parses the test cases, then iterates over each keyword, calling the code that implements the keyword. There isn't ever a stage where there's a completely pure python representation of a test case.
It is not possible to write tests in python, and have those tests run alongside traditional robot tests by the provided test runner. Like you said in your question, your only option is to put all of your logic for a single test case in a single keyword, and call that keyword from a test case.
It is possible to create and execute tests in python solely via the published API. This might not be what you're really asking for, because ultimately you're still creating keywords, you're just creating them via python.
from robot.api import TestSuite
suite = TestSuite('Activate Skynet')
suite.imports.library('OperatingSystem')
test = suite.tests.create('Should Activate Skynet', tags=['smoke'])
test.keywords.create('Set Environment Variable', args=['SKYNET', 'activated'], type='setup')
test.keywords.create('Environment Variable Should Be Set', args=['SKYNET'])
The above example was taken from here:
http://robot-framework.readthedocs.org/en/2.8.1/autodoc/robot.running.html