You cannot hardcode ExAws.request() if you want your code to use your mock for testing. You need to use some means of altering the module you‘re calling the request/1 function on.

I am doing something similiar also with Mox.

First i create a Impl file:

stats_logs_impl.ex

defmodule Stats.Logs.Impl do
  @callback fetch(Date.t(), String.t()) :: String.t()
end

then i create the API file:

stats_logs_impl.ex

defmodule Stats.Logs do
  @behaviour Stats.Logs.Impl

  @impl true
  def fetch(date, bucket), do: impl().fetch(date, bucket)

  defp impl, do: Application.get_env(:stats_logs, :impl, Stats.Logs.S3)
end

And finally the implementation

stats_logs_s3.ex

defmodule Stats.Logs.S3 do
  @behaviour Stats.Logs.Impl

  @impl true
  def fetch(%Date{} = date, bucket) do
    ...
  end
end

With this setup i can easily mock the module Stats.Log and its function fetch/2.

For those who are still wondering, yes it’s possible to use mock with ExAws without wrapping ExAws.request/2 in another module in order to mock it.

To do so, I read how they are testing their own module : ExAws Github

In fact, they are mocking the HTTP client underneath.
We can do exactly the same thing and mock the HTTP client in our tests.

In the business logic code:

...
    response =
      bucket_name
      |> S3.get_object(filename_path)
      |> ExAws.request(get_ex_aws_request_config_override())
...
  def get_ex_aws_request_config_override,
    do: Application.get_env(:ex_aws, :request_config_override)

config.exs:

...
config :ex_aws,
  ...
  # note: it's a custom config, ex_aws will not take it in account directly
  request_config_override: %{}
...

test.exs:

...
config :ex_aws,
  request_config_override: %{
    http_client: ExAws.Request.HttpMock,
    access_key_id: "AKIAIOSFODNN7EXAMPLE",
    secret_access_key: "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY"
  }
...

In the test code:

stub(ExAws.Request.HttpMock, :request, fn
      _method, _url, _body, _headers, _opts -> 
           # whatever you want to return, example:
           {:ok, %{status_code: 200, body: ""}}
    end)

I hope it will help !