what's the use of `fetch_schema_from_transport` flag

[Vichoko]

Having this flag as true adds a huge instantiation overhead over the gql.Client
In the docs isn't clear what's the actual difference between having this flag as true or false.
So my question is: What's the difference between having this flag or not?

I understand it's a boolean to indicate if we want to fetch the schema from the transport using an introspection query. But why/how is this useful at all?

[leszekhanusz]

If a schema is available, gql will validate the queries locally before sending them to the backend.
This is explained in the schema validation part of the docs.

You'll also need a schema:

• to compose queries dynamically with the DSL module
• to serialize/parse custom scalars or enums

If you don't want to validate your queries and don't need those two features, then no schema is needed.

Using introspection with fetch_schema_from_transport=True can be useful during development when the schema of the backend changes a lot as it ensures that the client always have the latest schema available. Once you're in production, if you need a schema for the reasons above, then you're better to just download the schema and provide it as a file in your client.

Feel free to propose a PR to make the docs clearer about this.

[Vichoko]

Thanks for the explanation @leszekhanusz

I wonder what is the advantage of locally validating the queries?

I just made some tests to test the validation, and it seems the exceptions raised for invalid queries are the same while validating the schema or not.

[leszekhanusz]

I wonder what is the advantage of locally validating the queries?

Not much, I agree

I just made some tests to test the validation, and it seems the exceptions raised for invalid queries are the same while validating the schema or not.

Normally you should receive a GraphQLError if the validation is done locally and a TransportQueryError if the validation is done by the backend.