In Django, how does one filter a QuerySet with dynamic field lookups?

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Given a class:

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=20)

Is it possible, and if so how, to have a QuerySet that filters based on dynamic arguments? For example:

 # Instead of:
 Person.objects.filter(name__startswith="B")
 # ... and:
 Person.objects.filter(name__endswith="B")

 # ... is there some way, given:
 filter_by = "{0}__{1}".format("name", "startswith")
 filter_value = "B"

 # ... that you can run the equivalent of this?
 Person.objects.filter(filter_by=filter_value)
 # ... which will throw an exception, since ’filter_by’ is not
 # an attribute of ’Person’.

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