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I'm writing unit tests for a function that should get all valid attachments in email message. My function has the following behaviour:

def extract_attachments(local_file):
 email_file = open(local_file, "r")
 msg = email.message_from_file(email_file) 
 attchs = [x for x in msg.get_payload() if x.get_filename() is not None]
 for attch in attchs:
 filename = attch.get_filename()
 if filename not in VALID_ATTACHMENTS:
 raise Exception('Invalid attachment', filename) 
 return attchs

How can I make this function return some object with a function get_filename()?

I'm trying to patch it but I'm getting an error 'list' object has no attribute 'get_payload'.

@patch("mail_extractor.process_email.email")
@patch("builtins.open")
def test_extract_attachments(open, email):
 email.message_from_file.return_value = []
 email.get_payload.return_value = ['1.xml']
 result = process_email.extract_attachments('mock')
 assert len(result) == 1

The notation you've described is specific to objects, and what you're trying to give it is a method. You'll need to define a class:

class MyClass:
 def get_filename():
 # some logic

Then you'll be able to instantiate your object:

x = MyClass()
x.get_filename()

Does that answer your question?

The way you've configured your mocks isn't quite right. The message_from_file() call needs to return a message, whereas you're returning a list - hence the error you're seeing.

You should set up a mock message along with a mock attachment in order to satisfy the calls the code expects to make.

For example:

@patch("mail_extractor.process_email.email")
@patch("builtins.open")
def test_extract_attachments(open, email):
 # Configure the mock message and its attachment
 mock_message = Mock()
 mock_attachment = Mock()
 mock_attachment.get_filename.return_value = '1.xml'
 mock_message.get_payload.return_value = [mock_attachment]

 # Configure message_from_file to return the mock message
 email.message_from_file.return_value = mock_message

 result = process_email.extract_attachments('mock')
 assert len(result) == 1