Change the string representation of a Class in Python

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Borislav Hadzhiev

Last updated: Sep 4, 2022

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Change the string representation of a Class in Python #

Use the __str__() method to change the string representation of a class. The __str__() method is called by str(object) and the built-in format() and print() functions and returns the informal string representation of the object.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __str__(self): return f'Employee name: {self.name}' emp = Employee('Alice', 100) print(emp) # 👉️ Employee name: Alice

We defined the __str__() method on the class to change its string representation.

The __str__() method is called by str(object) and the built-in format() and print() functions and returns the informal string representation of the object.

The print() function returns the output of the __str__() method.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __str__(self): return f'Employee name: {self.name}' emp = Employee('Alice', 100) print(emp) # 👉️ Employee name: Alice

Make sure to return a string from the __str__() method, otherwise a TypeError is raised.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __str__(self): # 👇️ returned integer but must be string return self.salary emp = Employee('Alice', 100) # ⛔️ TypeError: __str__ returned non-string (type int) print(emp)

If you need to return an integer, use the str() class to convert it to a string.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __str__(self): # 👇️ convert integer to string return str(self.salary) emp = Employee('Alice', 100) print(emp) # 👉️ 100

The __str__() method is called if you use the object in a formatted string literal or with the str.format() method.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __str__(self): return str(self.salary) emp = Employee('Alice', 100) result = f'Salary: {emp}' print(result) # 👉️ Salary: 100

The __str__() method should return a string that is a human-readable representation of the object.

There is also a __repr__() method that can be used in a similar way.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __repr__(self): return self.name emp = Employee('Alice', 100) print(emp) # 👉️ Alice print(repr(emp)) # 👉️ Alice

The __repr__ method is called by the repr() function and is usually used to get a string that can be used to rebuild the object using the eval() function.

If the class doesn't have the __str__() method defined, but has __repr__() defined, the output of __repr__() is used instead.

main.py
class Employee(): def __init__(self, name, salary): self.name = name self.salary = salary def __repr__(self): return self.name emp = Employee('Alice', 100) result = f'Employee name: {emp}' print(result) # 👉️ Employee name: Alice

A good way to illustrate the difference between __str__() and __repr__() is to use the datetime module.

main.py
import datetime # 👇️ using __str__() print(datetime.datetime.now()) # 👉️ 2022-09-04 16:47:40.839871 # 👇️ using __repr__() # 👉️ datetime.datetime(2022, 9, 4, 16, 47, 57, 981219) print(repr(datetime.datetime.now())) result = eval('datetime.datetime(2022, 9, 4, 16, 47, 57, 981219)') print(result) # 👉️ 2022-09-04 16:47:57.981219

When we used the print() function, the __str__() method in the datetime class got called and returned a human-readable representation of the date and time.

When we used the repr() function, the __repr__() method of the class got called and returned a string that can be used to recreate the same state of the object.

We passed the string to the eval() function and created a datetime object with the same state.

Note that implementing the __repr__() method in this way is not always necessary or possible.

Having the __str__() method return a human-readable string is sufficient most of the time.

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