How to print instances of a class using print()?

Ash_1630 0
Viewed 1167743

When I try to print an instance of a class, I get an output like this:

>>> class Test():
...     def __init__(self):
...         self.a = 'foo'
...
>>> print(Test())
<__main__.Test object at 0x7fc9a9e36d60>

How can I can define the printing behaviour (or the string representation) of a class and its instances? For example, referring to the above code, how can I modify the Test class so that printing an instance shows the a value?

object printing class python
12 Answers

If you're in a situation like @Keith you could try:

print(a.__dict__)

It goes against what I would consider good style but if you're just trying to debug then it should do what you want.

user_105530980

Simple. In the print, do:

print(foobar.__dict__)

as long as the constructor is

__init__
user_111860670

__repr__ and __str__ are already mentioned in many answers. I just want to add that if you are too lazy to add these magic functions to your class, you can use objprint. A simple decorator @add_objprint will help you add the __str__ method to your class and you can use print for the instance. Of course if you like, you can also use objprint function from the library to print any arbitrary objects in human readable format.

from objprint import add_objprint

class Position:
    def __init__(self, x, y):
        self.x = x
        self.y = y

@add_objprint
class Player:
    def __init__(self):
        self.name = "Alice"
        self.age = 18
        self.items = ["axe", "armor"]
        self.coins = {"gold": 1, "silver": 33, "bronze": 57}
        self.position = Position(3, 5)

print(Player())

The output is like

<Player
  .name = 'Alice',
  .age = 18,
  .items = ['axe', 'armor'],
  .coins = {'gold': 1, 'silver': 33, 'bronze': 57},
  .position = <Position
    .x = 3,
    .y = 5
  >
>
user_146285770

Even though this is an older post, there is also a very convenient method introduced in dataclasses (as of Python 3.7). Besides other special functions such as __eq__ and __hash__, it provides a __repr__ function for class attributes. You example would then be:

from dataclasses import dataclass, field
@dataclass
class Test:
    a: str = field(default="foo")
    b: str = field(default="bar")

t = Test()
print(t) 
# prints Test(a='foo', b='bar')

If you want to hide a certain attribute from being outputted, you can set the field decorator parameter repr to False:

@dataclass
class Test:
    a: str = field(default="foo")
    b: str = field(default="bar", repr=False)

t = Test()
print(t) 
# prints Test(a='foo')
user_104234810
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