python Protocols are Go interfaces

This week I successfully used python’s Protocol to fake a dependency on Hashicorp’s Vault.

Background:

• we need Vault access to save some data
• init and sidecar must be running for the pod to be created
• in development we don’t need access to the real vault
• in development access to the real vault crosses network boundaries via indirection

Instead, we have decided to use a verified fake and build an interface which we can use to swap out the real and fake Vault when required.

After a suggestion from a colleague about Protocol, and some light reading, this process was super simple. It works almost exactly the same as a Go interface whereby you define the interface and other classes implement it.

Example:

class SecretStore(Protocol):
  def save(self):
    pass

class VaultStore:
  def save(self):
    print("vault store save")

class FailStore:
  def no_save(self):
    print("not implemented")

class Saver:
  def do_save(self, obj: SecretStore):
    return obj.save()

def main():
  saver = Saver()
  saver.do_save(VaultStore())
  saver.do_save(FailStore()) # <-- AttributeError: BrokenStore object has no attribute 'save'

if __name__ == "__main__":
  main()

Tags:

#python #interfaces #programming