


What is a forward reference in Python type hints for classes?
Jul 09, 2025 am 01:46 AMForward references in Python allow referencing classes that are not yet defined by using quoted type names. They solve the issue of mutual class references like User and Profile where one class is not yet defined when referenced. By enclosing the class name in quotes (e.g., 'Profile'), Python delays resolving the reference until it's needed. This technique should only be used when necessary, such as in circular dependencies, and can also be applied in function signatures. Starting from Python 3.11 or with from __future__ import annotations, quotes are unnecessary. Common mistakes include typos in quoted names, overuse, and not defining the referenced class later. Using tools like mypy helps ensure correctness.
When you're working with type hints in Python, especially when dealing with classes that reference each other, you might run into a problem: one class refers to another that hasn't been defined yet. That's where a forward reference comes in.

A forward reference is basically a way to tell Python, "This type will exist later, so don’t worry about it right now." You do this by putting the type name in quotes, like 'ClassName'
, instead of using it directly.
Why you need forward references
Let’s say you have two classes that refer to each other — for example, User
and Profile
. In Python, code runs top to bottom. So if User
has a field that refers to Profile
, and Profile
also has a field that refers back to User
, you’ll hit an error if you try to use the class name directly before it's defined.

Here’s what that looks like without quotes (and would cause a NameError
):
class User: profile: Profile # Error! Profile isn't defined yet class Profile: user: User
By using a string instead, like 'Profile'
, you avoid that issue:

class User: profile: 'Profile' # This works! class Profile: user: User
Python understands that 'Profile'
is a type that will be defined later.
How to use them correctly
Using forward references is straightforward — just wrap the class name in quotes. But there are a few things to keep in mind:
- Only use them when necessary, like when two classes depend on each other.
- If the class is already defined above, no need for quotes.
- They work not just in class attributes, but also in function signatures and return types.
Examples:
def get_user() -> 'User': ... class User: friend: 'User' # Refers to same class
Also, starting from Python 3.11 (or using from __future__ import annotations
in Python 3.7 ), you can skip the quotes entirely because annotations are automatically treated as forward references. But if you’re supporting older versions or writing code that needs to run across multiple Python versions, using quotes is still safer.
Common mistakes and how to avoid them
Sometimes people forget that forward references are only placeholders. Here are a few gotchas:
- ? Using invalid class names inside quotes — Python won't catch typos until runtime.
- ? Forgetting to define the actual class later — your code will crash when it tries to resolve the reference.
- ? Overusing them even when the class is already defined — it's unnecessary and can confuse readers.
If you're using tools like mypy or Pyright for static typing, they usually handle quoted references well, but it’s good to test with your own setup.
So, to sum up:
- Use
'ClassName'
when a class hasn't been defined yet - Don’t quote class names once both are defined in the right order
- Know your Python version and tooling support
基本上就這些。
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