Infinite recursion in docs - Nesting a schema within itself

[piyush-jaiswal]

Link: marshmallow.readthedocs.io/en/stable/nesting.html#nesting-a-schema-within-itself

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    # Use the 'exclude' argument to avoid infinite recursion
    employer = fields.Nested(lambda: UserSchema(exclude=("employer",)))
    friends = fields.List(fields.Nested(lambda: UserSchema()))


user = User("Steve", "steve@example.com")
user.friends.append(User("Mike", "mike@example.com"))
user.friends.append(User("Joe", "joe@example.com"))
user.employer = User("Dirk", "dirk@example.com")
result = UserSchema().dump(user)
pprint(result, indent=2)
# {
#     "name": "Steve",
#     "email": "steve@example.com",
#     "friends": [
#         {
#             "name": "Mike",
#             "email": "mike@example.com",
#             "friends": [],
#             "employer": null
#         },
#         {
#             "name": "Joe",
#             "email": "joe@example.com",
#             "friends": [],
#             "employer": null
#         }
#     ],
#     "employer": {
#         "name": "Dirk",
#         "email": "dirk@example.com",
#         "friends": []
#     }
# }

---

While the example usage itself doesn't run into infinite recursion, the schema is prone to it:

user = User("Steve", "steve@example.com")
dirk = User("Dirk", "dirk@example.com")
user.employer = dirk
dirk.friends.append(user)

result = UserSchema().dump(user)
pprint(result, indent=2)

user = User("Steve", "steve@example.com")
mike = User("Mike", "mike@example.com")
joe = User("Joe", "joe@example.com")

user.friends.append(mike)
mike.friends.append(user)
user.friends.append(joe)
joe.friends.append(user)

dirk = User("Dirk", "dirk@example.com")
user.employer = dirk
dirk.friends.append(user)

result = UserSchema().dump(user)
pprint(result, indent=2)

---

Possible solutions:

1. Make either relationship a leaf node

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    # Use the 'exclude' argument to avoid infinite recursion
    employer = fields.Nested(lambda: UserSchema(exclude=("employer", "friends")))
    friends = fields.List(fields.Nested(lambda: UserSchema(exclude=("friends",))))

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    # Use the 'exclude' argument to avoid infinite recursion
    employer = fields.Nested(lambda: UserSchema(exclude=("employer",)))
    friends = fields.List(fields.Nested(lambda: UserSchema(exclude=("employer", "friends"))))

2. Make both relationships leaf nodes

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    # Use the 'exclude' argument to avoid infinite recursion
    employer = fields.Nested(lambda: UserSchema(exclude=("employer", "friends")))
    friends = fields.List(fields.Nested(lambda: UserSchema(exclude=("employer", "friends"))))

3. Use a single relationship

class UserSchema(Schema):
    name = fields.String()
    email = fields.Email()
    # Use the 'exclude' argument to avoid infinite recursion
    friends = fields.List(fields.Nested(lambda: UserSchema(exclude=("friends",))))

Happy to contribute if accepted!