Python 的自我与实例

Question

What is the difference between the self and instance keywords in Python 3?

I see code like,

def update(self, instance, validated_data):
    """
    Update and return an existing `Snippet` instance, given the validated data.
    """
    instance.title = validated_data.get('title', instance.title)
    instance.code = validated_data.get('code', instance.code)
    instance.linenos = validated_data.get('linenos', instance.linenos)
    instance.language = validated_data.get('language', instance.language)
    instance.style = validated_data.get('style', instance.style)
    instance.save()
    return instance

Answer 1

The snippet is a bit short but instance is not a keyword (neither self , that is just convention).

It is an argument to another instance of another (maybe same) class.

Answer 2

The question is rather generic, but let me see if I can shed some light on it for you:

self refers to the class(by convention, not a keyword) of which update is a part. The class has variables and methods and you can refer to these with the self keyword(not a reserved keyword) by calling self.update(instance, validated_data)

In the case of the snippet above, self refers to the class. instance likely refers to some model instance "the big hint is the instance.save() and validated_data is a dictionary or class object with attributes you are get tting and assigning to instance attributes before saving them

Hope this helps

Answer 3

Neither self nor instance are keywords in Python. The identifier self is used by convention as the first parameter of instance methods in a class. The object instance on which a method is called is automatically passed in as the first parameter.

In the above snippet, update is most probably a method of some class and self seems to be the conventional first parameter as described above. The second parameter instance is just another parameter and the name instance does not have any significance in Python.

Answer 4

You are refering to the implementation of a Serializer class in Django REST framework, that serializes data into Json with help of a Model object. This class has a method called 'update', which takes the Snippet Model object 'instance' as an input, and then updates this object with the validated_data. Like it was mentioned before 'instance' is not a reserved keyword in Python.

A serializer class is very similar to a Django Form class, and includes similar validation flags on the various fields, such as required, max_length and default.

In order get clearer idea I am sharing the complete class implementation.

class SnippetSerializer(serializers.Serializer):

    id = serializers.IntegerField(read_only=True)
    title = serializers.CharField(required=False, allow_blank=True, max_length=100)
    code = serializers.CharField(style={'base_template': 'textarea.html'})
    linenos = serializers.BooleanField(required=False)
    language = serializers.ChoiceField(choices=LANGUAGE_CHOICES, default='python')
    style = serializers.ChoiceField(choices=STYLE_CHOICES, default='friendly')

    def create(self, validated_data):
        """
        Create and return a new `Snippet` instance, given the validated data.
        """
        return Snippet.objects.create(**validated_data)

    def update(self, instance, validated_data):
        """
        Update and return an existing `Snippet` instance, given the validated data.
        """
        instance.title = validated_data.get('title', instance.title)
        instance.code = validated_data.get('code', instance.code)
        instance.linenos = validated_data.get('linenos', instance.linenos)
        instance.language = validated_data.get('language', instance.language)
        instance.style = validated_data.get('style', instance.style)
        instance.save()
        return instance