Python: How to write a lambda that accesses instance variables

Question

I am writing a class that stores credentials. The credential can have a password_getter attribute which stores a lambda that will get the password on-demand. This getter needs access to self.username in order to function. How can this be accomplished? Consider the following setup:

class Credential:
    """Defines one username and password combination, and how to get them."""

    def __init__(self, username: str, password_getter: callable):
        self.username = username
        self._password_getter = password_getter

    @property
    def password(self) -> str:
        self.password = self._password_getter()
        return self.password

Then we define a new credential:

cred = Credential(
    username="test",
    password_getter=lambda self: self.username+"_pass",
)
assert cred.password == "test_pass"

The above is non-functional due to self not existing during definition, but it suggests what I'm trying to do: Access the current value of cred.username from inside the lambda.

For some context as to why this is happening; the function in production actually goes out to a password vault api and requests the password for the given username.

You need to pass self when calling the getter, or you use lambda: cred.username + '_pass'. — deceze
– deceze ♦, Commented Mar 25, 2022 at 9:11
The easiest way is to do : python username="test" cred = Credential( username="username", password_getter=lambda : username+"_pass", ) — Brinfer
– Brinfer, Commented Mar 25, 2022 at 9:15
The problem is that another part of the code could theoretically change the username later. The getter needs to be able to use the current username in its response. — Wyko ter Haar
– Wyko ter Haar, Commented Mar 25, 2022 at 9:32
so why isn't this fn just a method on Credential if it needs access to state stored on the instance? You can still call out to any 3rd party fn you need. — 2e0byo
– 2e0byo, Commented Mar 25, 2022 at 9:40
Because Credential just defines a framework for storing and retrieving usernames and passwords. The actual getter code varies significantly between credentials, ranging from checking environment variables to querying remote APIs. I could subclass Credential for each cred, but that just seemed bulky if I could use lambdas. — Wyko ter Haar
– Wyko ter Haar, Commented Mar 25, 2022 at 10:10

deceze · Accepted Answer · 2022-03-25 09:50:48Z

2

The Callable you pass in has no intrinsic connection to the instance, so your two options are to give it implicit or explicit access. Implicit would rely on simple closures:

cred = Credential(
    username="test",
    password_getter=lambda: cred.username + "_pass",
)

The problem here is if you reassign cred later and it refers to the wrong instance when trying to get the password.

The explicit alternative is:

cred = Credential(
    username="test",
    password_getter=lambda instance: instance.username + "_pass",
)

...
    @property
    def password(self) -> str:
        self.password = self._password_getter(self)

The class explicitly passes itself to the password getter. So it should be typed as Callable[[Credentials], str], i.e. something that receives an instance of the class as argument and is expected to return a string.

answered Mar 25, 2022 at 9:50

deceze♦

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1 Comment

Wyko ter Haar Over a year ago

Nice! I totally missed the obvious choice to pass self in self._password_getter(self). Thanks for that!

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