How to mock class attributes using pytest

Question

I have a class something like this.

class Upgrade:

    def __init__(self, ssh_client):
        self.ssh_client = ssh_client
        self.channel = self.ssh_client.invoke_shell(width=1000, height=1000)
        self.stdin = self.channel.makefile('wb')
        self.stdout = self.channel.makefile('r')


    def do_upgrade(self):

        # execute some commands on paramiko channel

        for line in self.stdout:
            
            if str(line).startswith("PLAY RECAP"):
                
                # do something

When I trying to mock self attribute called 'stdout' like this(using pytest),

def return_stdout(*args, **kwargs):
    stdout = "\n\rSome return value\n\r"
    return stdout
monkeypatch.setattr(Upgrade, 'stdout', return_stdout)

I'm getting following error.

>     monkeypatch.setattr(Upgrade, 'stdout', return_stdout)
E     AttributeError: <class 'Upgrade'> has no attribute 'stdout'
        

Then, How can I mock 'stdout' using pytest or pytest-mock?

Answer 1

the class does not have the attribute, you're setting stdout as an instance variable. And even if you'd mock it, you'd overwrite it in the constructor ( self.stdout = self.channel.makefile('r') ). Create a wrapper/property, and monkeypatch that instead:

class Upgrade:
    def __init__(self, ssh_client):
        self._stdout = self.channel.makefile('r')

    def get_stdout(self):
        return self._stdout

then in the test mock the encapsulating method instead:

monkeypatch.setattr(Upgrade, 'get_stdout', return_stdout)