I’m trying to write unit tests for a FastAPI application that interacts with an SFTP server. Specifically, I’m having trouble mocking the SFTP client object to test the get_folder_structure function. Here’s a simplified version of my code:

import stat

from fastapi import APIRouter, HTTPException, Query, Depends
import paramiko

# Function to get SFTP client
def get_sftp_client():
    try:
        pkey = paramiko.RSAKey.from_private_key_file("C:/Users/User/Desktop/test_rsa.key")
        transport = paramiko.Transport(("localhost", 3373))
        transport.connect(username="admin", password="admin", pkey=pkey)
        sftp = paramiko.SFTPClient.from_transport(transport)
        return sftp
    except Exception as e:
        raise HTTPException(status_code=500, detail=f"Failed to connect to SFTP server: {str(e)}")

# Function to get folder structure
def get_folder_structure(sftp, folder_path="."):
    folder_structure = {}
    try:
        for item in sftp.listdir_attr(folder_path):
            item_path = os.path.join(folder_path, item.filename)
            if stat.S_ISDIR(item.st_mode):
                folder_structure[item.filename] = get_folder_structure(sftp, item_path)
            else:
                folder_structure[item.filename] = None
        return folder_structure
    except Exception as e:
        raise HTTPException(status_code=500, detail=str(e))

# Route handler
@router.get("/folders")
def get_folders(sftp=Depends(get_sftp_client)):
    try:
        folder_structure = get_folder_structure(sftp)
        sftp.close()
        return {"folder_structure": folder_structure}
    except Exception as e:
        raise e

And here’s my attempt to mock the SFTP client in my unit tests:

from unittest.mock import MagicMock

from app.routers.six import get_folder_structure, get_sftp_client

@pytest.fixture
def mocked_sftp():
    sftp = MagicMock()
    sftp.listdir_attr.return_value = [
        MagicMock(filename="folder1", st_mode=16877),
        MagicMock(filename="file1.txt", st_mode=33188),
        MagicMock(filename="folder2", st_mode=16877),
    ]
    return sftp

def test_get_folder_structure_success(mocked_sftp):
    sftp = get_sftp_client()
    # Arrange
    expected_structure = {
        "folder1": {
            "file1.txt": None,
        },
        "folder2": None,
    }

    actual_structure = get_folder_structure(mocked_sftp)

    assert actual_structure == expected_structure

def test_get_folder_structure_exception(mocked_sftp):
    # Arrange
    mocked_sftp.listdir_attr.side_effect = Exception("Mocked exception")

    # Act & Assert
    with pytest.raises(Exception) as exc_info:
        get_folder_structure(mocked_sftp)

    assert str(exc_info.value) == "Mocked exception"

However, when I run these tests, I get an error indicating that the get_sftp_client function is not being mocked properly. How can I correctly mock the SFTP client in my unit tests to test the get_folder_structure function?

I’ve used the package sftpserver and you can run that via:
sftpserver -k “C:UsersUserDesktoptest_rsa.key” -l DEBUG

Thanks in advance for your help!