It is just a trick about reducing the number of parameters to send to function format. Let's assume we have a class like the following:
class Entity: def __init__(self, name, city): self.name = name self.city = city school = Entity("ENSAE", "Paris")
And we want to build a string like the following:
message = "school:{0} city:{1}".format(school.name, school.city) # two parameters
But it could be rewritten as:
# one parameter message = "school:{0.name} city:{0.city}".format(school)
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