Testing private functions in Elixir

testing_defps.ex

defmodule TestDefp do
  defmacro __using__(_) do
    quote do
      import Kernel, except: [defp: 2]
    end
  end

  # Let's redefine `defp/2` so that if MIX_ENV is `test`, the function will be
  # defined with `def/2` instead of `defp/2`.
  defmacro defp(fun, body) do
    def_macro = if function_exported?(Mix, :env, 0) && apply(Mix, :env, [:test]), do: :def, else: :defp
    quote, do: Kernel.unquote(def_macro)(unquote(fun), unquote(body))
  end
end

defmodule MyModule do
  use TestDefp
  
  defp my_fun() do
    :ok
  end
end

# Now you can test `MyModule.my_fun/0` when MIX_ENV=test :).
# By the way, strive not to test private functions as it may
# be a sign of code smell. You can always export them into a
# separate module and use `@moduledoc false` or simply make
# them public with `@doc false`.

Love this but unfortunately it's invasive as you need to change the code under test.

Excellent gist, but I had to make a small change to get it to work. Within the quote do block, after:

import Kernel, except [defp:2]

You have to import this module (TestDefp) to get the macro to kick in:

import __MODULE__

Without this change, the compiler squawks because defp is undefined after the "use TestDefp" statement in the module to test.
Thanks.