Factory pattern
- Python course
- Intermediate
- Lesson 26/116
- 5 min
Learn the factory pattern with @classmethod.
The factory pattern
The factory pattern lets you build an instance of a class from a different input. Usually a @classmethod returns a new instance; its name starts with from_....
Example: class User:\n @classmethod\n def from_birth_year(cls, name, year):\n return cls(name, 2025 - year). This method computes the age from the birth year and builds the instance. The caller only passes name and year; the age math is inside the method. This simplifies the call.
The from_ naming convention
Factory methods are usually named with a from_ prefix: from_list, from_string, from_dict. This helps the caller see at once that it is an alternate constructor.
Example: Counter.from_list([1, 2, 3]) builds an instance from a list. User.from_birth_year("Alex", 1990) builds one from a birth year. Readability improves this way; the call says what it does.
Practice Add a factory to User
Define the User class. Add def __init__(self, name, age): with self.name = name and self.age = age in the body. Add @classmethod and def from_birth_year(cls, name, birth_year):; in the body write return cls(name, 2025 - birth_year). Then create an instance with u = User.from_birth_year("Alex", 1990) and write print(u.age).
Starter code
# Add a factory to the User classSample solution
# Add a factory to the User class
class User:
def __init__(self, name, age):
self.name = name
self.age = age
@classmethod
def from_birth_year(cls, name, birth_year):
return cls(name, 2025 - birth_year)
u = User.from_birth_year("Alex", 1990)
print(u.age)Expected output
35
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the factory concepts
| Term | Meaning |
|---|---|
from_birth_year |
Factory method |
cls(name, ...) |
New instance |
2025 - year |
Age math |
2 What does a factory method return?
- The class name
- An instance Correct answer
- A method
- None
Explanation: A factory method returns a new class instance.
3 Complete the factory return
@classmethod
def from_birth_year(cls, name, year):
return cls(name, 2025 - ____) @classmethod
def from_birth_year(cls, name, year):
return cls(name, 2025 - year)Explanation: The year parameter is used in the age math.