Django技术疑问：get()方法是否为惰性执行？以及Django是否知晓各模型的实例数量？

Great questions—let's tackle them one by one to clear up the confusion!

1. Why does Model.objects.get(pk=1) throw DoesNotExist immediately, even when just assigning to a variable? #

You're totally right that regular Django QuerySets are lazy—methods like filter() or exclude() just build up a query plan without hitting the database until the QuerySet is evaluated (like looping over it, converting to a list, or printing it). But get() is different: it's not designed to return a QuerySet. Instead, its job is to fetch a single matching instance right away.

Under the hood, get() works by:

• Creating a filtered QuerySet (like Model.objects.filter(pk=1))
• Immediately evaluating that QuerySet to check for results
• If no matches are found, it raises the DoesNotExist exception you saw
• If multiple matches exist, it raises MultipleObjectsReturned instead

Since get() has to return an actual model instance (not just a query plan), it can't be lazy. The database query happens the moment you call get()—even if you're just assigning the result to a variable. That's why you see the exception immediately when the record doesn't exist.

2. Does Django automatically know the number of instances for each model? #

Short answer: No, Django doesn't track instance counts in real-time or keep a cached total by default. But there are easy, efficient ways to get this number when you need it:

• Use Model.objects.count(): This executes a lightweight SELECT COUNT(*) SQL query directly against the database, which is much faster than loading all instances into memory (like len(Model.objects.all()) would do). It's the recommended way to get the total number of instances for a model.
• For frequent count checks (and if your data doesn't change constantly): You can cache the count using Django's built-in caching framework, or add a custom counter field to your model (e.g., instance_count). Then use Django signals (post_save and post_delete) to increment/decrement this field whenever an instance is created or deleted. This lets you avoid hitting the database for a count every time.

内容的提问来源于stack exchange，提问作者Omar AlHadidi