Related
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 make it so that the print will show something custom (e.g. something that includes the a attribute value)? That is, how can I can define how the instances of the class will appear when printed (their string representation)?
See How can I choose a custom string representation for a class itself (not instances of the class)? if you want to define the behaviour for the class itself (in this case, so that print(Test) shows something custom, rather than <class __main__.Test> or similar). (In fact, the technique is essentially the same, but trickier to apply.)
>>> class Test:
... def __repr__(self):
... return "Test()"
... def __str__(self):
... return "member of Test"
...
>>> t = Test()
>>> t
Test()
>>> print(t)
member of Test
The __str__ method is what gets called happens when you print it, and the __repr__ method is what happens when you use the repr() function (or when you look at it with the interactive prompt).
If no __str__ method is given, Python will print the result of __repr__ instead. If you define __str__ but not __repr__, Python will use what you see above as the __repr__, but still use __str__ for printing.
As Chris Lutz explains, this is defined by the __repr__ method in your class.
From the documentation of repr():
For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.
Given the following class Test:
class Test:
def __init__(self, a, b):
self.a = a
self.b = b
def __repr__(self):
return f"<Test a:{self.a} b:{self.b}>"
def __str__(self):
return f"From str method of Test: a is {self.a}, b is {self.b}"
..it will act the following way in the Python shell:
>>> t = Test(123, 456)
>>> t
<Test a:123 b:456>
>>> print(repr(t))
<Test a:123 b:456>
>>> print(t)
From str method of Test: a is 123, b is 456
>>> print(str(t))
From str method of Test: a is 123, b is 456
If no __str__ method is defined, print(t) (or print(str(t))) will use the result of __repr__ instead
If no __repr__ method is defined then the default is used, which is roughly equivalent to:
def __repr__(self):
cls = self.__class__
return f"<{cls.__module_}.{cls.__qualname__} object at {id(self)}>"
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.
A generic way that can be applied to any class without specific formatting could be done as follows:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + ": " + str(self.__dict__)
And then,
elem = Element('my_name', 'some_symbol', 3)
print(elem)
produces
__main__.Element: {'symbol': 'some_symbol', 'name': 'my_name', 'number': 3}
A prettier version of response by #user394430
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + '\n'+ '\n'.join(('{} = {}'.format(item, self.__dict__[item]) for item in self.__dict__))
elem = Element('my_name', 'some_symbol', 3)
print(elem)
Produces visually nice list of the names and values.
<class '__main__.Element'>
name = my_name
symbol = some_symbol
number = 3
An even fancier version (thanks Ruud) sorts the items:
def __str__(self):
return str(self.__class__) + '\n' + '\n'.join((str(item) + ' = ' + str(self.__dict__[item]) for item in sorted(self.__dict__)))
Simple. In the print, do:
print(foobar.__dict__)
as long as the constructor is
__init__
For Python 3:
If the specific format isn't important (e.g. for debugging) just inherit from the Printable class below. No need to write code for every object.
Inspired by this answer
class Printable:
def __repr__(self):
from pprint import pformat
return "<" + type(self).__name__ + "> " + pformat(vars(self), indent=4, width=1)
# Example Usage
class MyClass(Printable):
pass
my_obj = MyClass()
my_obj.msg = "Hello"
my_obj.number = "46"
print(my_obj)
Just to add my two cents to #dbr's answer, following is an example of how to implement this sentence from the official documentation he's cited:
"[...] to return a string that would yield an object with the same value when passed to eval(), [...]"
Given this class definition:
class Test(object):
def __init__(self, a, b):
self._a = a
self._b = b
def __str__(self):
return "An instance of class Test with state: a=%s b=%s" % (self._a, self._b)
def __repr__(self):
return 'Test("%s","%s")' % (self._a, self._b)
Now, is easy to serialize instance of Test class:
x = Test('hello', 'world')
print 'Human readable: ', str(x)
print 'Object representation: ', repr(x)
print
y = eval(repr(x))
print 'Human readable: ', str(y)
print 'Object representation: ', repr(y)
print
So, running last piece of code, we'll get:
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
But, as I said in my last comment: more info is just here!
You need to use __repr__. This is a standard function like __init__.
For example:
class Foobar():
"""This will create Foobar type object."""
def __init__(self):
print "Foobar object is created."
def __repr__(self):
return "Type what do you want to see here."
a = Foobar()
print a
__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
>
>
There are already a lot of answers in this thread but none of them particularly helped me, I had to work it out myself, so I hope this one is a little more informative.
You just have to make sure you have parentheses at the end of your class, e.g:
print(class())
Here's an example of code from a project I was working on:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return "{}: {}\nAtomic Number: {}\n".format(self.name, self.symbol, self.number
class Hydrogen(Element):
def __init__(self):
super().__init__(name = "Hydrogen", symbol = "H", number = "1")
To print my Hydrogen class, I used the following:
print(Hydrogen())
Please note, this will not work without the parentheses at the end of Hydrogen. They are necessary.
Hope this helps, let me know if you have anymore questions.
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')
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 make it so that the print will show something custom (e.g. something that includes the a attribute value)? That is, how can I can define how the instances of the class will appear when printed (their string representation)?
See How can I choose a custom string representation for a class itself (not instances of the class)? if you want to define the behaviour for the class itself (in this case, so that print(Test) shows something custom, rather than <class __main__.Test> or similar). (In fact, the technique is essentially the same, but trickier to apply.)
>>> class Test:
... def __repr__(self):
... return "Test()"
... def __str__(self):
... return "member of Test"
...
>>> t = Test()
>>> t
Test()
>>> print(t)
member of Test
The __str__ method is what gets called happens when you print it, and the __repr__ method is what happens when you use the repr() function (or when you look at it with the interactive prompt).
If no __str__ method is given, Python will print the result of __repr__ instead. If you define __str__ but not __repr__, Python will use what you see above as the __repr__, but still use __str__ for printing.
As Chris Lutz explains, this is defined by the __repr__ method in your class.
From the documentation of repr():
For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.
Given the following class Test:
class Test:
def __init__(self, a, b):
self.a = a
self.b = b
def __repr__(self):
return f"<Test a:{self.a} b:{self.b}>"
def __str__(self):
return f"From str method of Test: a is {self.a}, b is {self.b}"
..it will act the following way in the Python shell:
>>> t = Test(123, 456)
>>> t
<Test a:123 b:456>
>>> print(repr(t))
<Test a:123 b:456>
>>> print(t)
From str method of Test: a is 123, b is 456
>>> print(str(t))
From str method of Test: a is 123, b is 456
If no __str__ method is defined, print(t) (or print(str(t))) will use the result of __repr__ instead
If no __repr__ method is defined then the default is used, which is roughly equivalent to:
def __repr__(self):
cls = self.__class__
return f"<{cls.__module_}.{cls.__qualname__} object at {id(self)}>"
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.
A generic way that can be applied to any class without specific formatting could be done as follows:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + ": " + str(self.__dict__)
And then,
elem = Element('my_name', 'some_symbol', 3)
print(elem)
produces
__main__.Element: {'symbol': 'some_symbol', 'name': 'my_name', 'number': 3}
A prettier version of response by #user394430
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + '\n'+ '\n'.join(('{} = {}'.format(item, self.__dict__[item]) for item in self.__dict__))
elem = Element('my_name', 'some_symbol', 3)
print(elem)
Produces visually nice list of the names and values.
<class '__main__.Element'>
name = my_name
symbol = some_symbol
number = 3
An even fancier version (thanks Ruud) sorts the items:
def __str__(self):
return str(self.__class__) + '\n' + '\n'.join((str(item) + ' = ' + str(self.__dict__[item]) for item in sorted(self.__dict__)))
Simple. In the print, do:
print(foobar.__dict__)
as long as the constructor is
__init__
For Python 3:
If the specific format isn't important (e.g. for debugging) just inherit from the Printable class below. No need to write code for every object.
Inspired by this answer
class Printable:
def __repr__(self):
from pprint import pformat
return "<" + type(self).__name__ + "> " + pformat(vars(self), indent=4, width=1)
# Example Usage
class MyClass(Printable):
pass
my_obj = MyClass()
my_obj.msg = "Hello"
my_obj.number = "46"
print(my_obj)
Just to add my two cents to #dbr's answer, following is an example of how to implement this sentence from the official documentation he's cited:
"[...] to return a string that would yield an object with the same value when passed to eval(), [...]"
Given this class definition:
class Test(object):
def __init__(self, a, b):
self._a = a
self._b = b
def __str__(self):
return "An instance of class Test with state: a=%s b=%s" % (self._a, self._b)
def __repr__(self):
return 'Test("%s","%s")' % (self._a, self._b)
Now, is easy to serialize instance of Test class:
x = Test('hello', 'world')
print 'Human readable: ', str(x)
print 'Object representation: ', repr(x)
print
y = eval(repr(x))
print 'Human readable: ', str(y)
print 'Object representation: ', repr(y)
print
So, running last piece of code, we'll get:
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
But, as I said in my last comment: more info is just here!
You need to use __repr__. This is a standard function like __init__.
For example:
class Foobar():
"""This will create Foobar type object."""
def __init__(self):
print "Foobar object is created."
def __repr__(self):
return "Type what do you want to see here."
a = Foobar()
print a
__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
>
>
There are already a lot of answers in this thread but none of them particularly helped me, I had to work it out myself, so I hope this one is a little more informative.
You just have to make sure you have parentheses at the end of your class, e.g:
print(class())
Here's an example of code from a project I was working on:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return "{}: {}\nAtomic Number: {}\n".format(self.name, self.symbol, self.number
class Hydrogen(Element):
def __init__(self):
super().__init__(name = "Hydrogen", symbol = "H", number = "1")
To print my Hydrogen class, I used the following:
print(Hydrogen())
Please note, this will not work without the parentheses at the end of Hydrogen. They are necessary.
Hope this helps, let me know if you have anymore questions.
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')
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 make it so that the print will show something custom (e.g. something that includes the a attribute value)? That is, how can I can define how the instances of the class will appear when printed (their string representation)?
See How can I choose a custom string representation for a class itself (not instances of the class)? if you want to define the behaviour for the class itself (in this case, so that print(Test) shows something custom, rather than <class __main__.Test> or similar). (In fact, the technique is essentially the same, but trickier to apply.)
>>> class Test:
... def __repr__(self):
... return "Test()"
... def __str__(self):
... return "member of Test"
...
>>> t = Test()
>>> t
Test()
>>> print(t)
member of Test
The __str__ method is what gets called happens when you print it, and the __repr__ method is what happens when you use the repr() function (or when you look at it with the interactive prompt).
If no __str__ method is given, Python will print the result of __repr__ instead. If you define __str__ but not __repr__, Python will use what you see above as the __repr__, but still use __str__ for printing.
As Chris Lutz explains, this is defined by the __repr__ method in your class.
From the documentation of repr():
For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.
Given the following class Test:
class Test:
def __init__(self, a, b):
self.a = a
self.b = b
def __repr__(self):
return f"<Test a:{self.a} b:{self.b}>"
def __str__(self):
return f"From str method of Test: a is {self.a}, b is {self.b}"
..it will act the following way in the Python shell:
>>> t = Test(123, 456)
>>> t
<Test a:123 b:456>
>>> print(repr(t))
<Test a:123 b:456>
>>> print(t)
From str method of Test: a is 123, b is 456
>>> print(str(t))
From str method of Test: a is 123, b is 456
If no __str__ method is defined, print(t) (or print(str(t))) will use the result of __repr__ instead
If no __repr__ method is defined then the default is used, which is roughly equivalent to:
def __repr__(self):
cls = self.__class__
return f"<{cls.__module_}.{cls.__qualname__} object at {id(self)}>"
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.
A generic way that can be applied to any class without specific formatting could be done as follows:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + ": " + str(self.__dict__)
And then,
elem = Element('my_name', 'some_symbol', 3)
print(elem)
produces
__main__.Element: {'symbol': 'some_symbol', 'name': 'my_name', 'number': 3}
A prettier version of response by #user394430
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return str(self.__class__) + '\n'+ '\n'.join(('{} = {}'.format(item, self.__dict__[item]) for item in self.__dict__))
elem = Element('my_name', 'some_symbol', 3)
print(elem)
Produces visually nice list of the names and values.
<class '__main__.Element'>
name = my_name
symbol = some_symbol
number = 3
An even fancier version (thanks Ruud) sorts the items:
def __str__(self):
return str(self.__class__) + '\n' + '\n'.join((str(item) + ' = ' + str(self.__dict__[item]) for item in sorted(self.__dict__)))
Simple. In the print, do:
print(foobar.__dict__)
as long as the constructor is
__init__
For Python 3:
If the specific format isn't important (e.g. for debugging) just inherit from the Printable class below. No need to write code for every object.
Inspired by this answer
class Printable:
def __repr__(self):
from pprint import pformat
return "<" + type(self).__name__ + "> " + pformat(vars(self), indent=4, width=1)
# Example Usage
class MyClass(Printable):
pass
my_obj = MyClass()
my_obj.msg = "Hello"
my_obj.number = "46"
print(my_obj)
Just to add my two cents to #dbr's answer, following is an example of how to implement this sentence from the official documentation he's cited:
"[...] to return a string that would yield an object with the same value when passed to eval(), [...]"
Given this class definition:
class Test(object):
def __init__(self, a, b):
self._a = a
self._b = b
def __str__(self):
return "An instance of class Test with state: a=%s b=%s" % (self._a, self._b)
def __repr__(self):
return 'Test("%s","%s")' % (self._a, self._b)
Now, is easy to serialize instance of Test class:
x = Test('hello', 'world')
print 'Human readable: ', str(x)
print 'Object representation: ', repr(x)
print
y = eval(repr(x))
print 'Human readable: ', str(y)
print 'Object representation: ', repr(y)
print
So, running last piece of code, we'll get:
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
Human readable: An instance of class Test with state: a=hello b=world
Object representation: Test("hello","world")
But, as I said in my last comment: more info is just here!
You need to use __repr__. This is a standard function like __init__.
For example:
class Foobar():
"""This will create Foobar type object."""
def __init__(self):
print "Foobar object is created."
def __repr__(self):
return "Type what do you want to see here."
a = Foobar()
print a
__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
>
>
There are already a lot of answers in this thread but none of them particularly helped me, I had to work it out myself, so I hope this one is a little more informative.
You just have to make sure you have parentheses at the end of your class, e.g:
print(class())
Here's an example of code from a project I was working on:
class Element:
def __init__(self, name, symbol, number):
self.name = name
self.symbol = symbol
self.number = number
def __str__(self):
return "{}: {}\nAtomic Number: {}\n".format(self.name, self.symbol, self.number
class Hydrogen(Element):
def __init__(self):
super().__init__(name = "Hydrogen", symbol = "H", number = "1")
To print my Hydrogen class, I used the following:
print(Hydrogen())
Please note, this will not work without the parentheses at the end of Hydrogen. They are necessary.
Hope this helps, let me know if you have anymore questions.
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')
I am trying to do the following:
node = ''.join(c_info['in']+c_info['out'])
TypeError: sequence item 0: expected str instance, Resistor found
Is there a dunder method that I can add/modify so that a string join will work with instances of that class? Or do I need to manually do the string formatting? Otherwise, what I'm doing is:
node = ''.join([str(item) for item in (c_info['in'] + c_info['out'])])
You can create a + operator in your Resistor to explicitly return the kind of type you want. (Which, incidentally, also means you do not need join anymore.)
class Foo:
def __init__(self, initial_value = 0):
self.bar = initial_value
def __add__(self, other):
return str(self.bar)+str(other.bar)
foo1 = Foo(10)
foo2 = Foo(20)
print (foo1+foo2)
Result (a regular string):
1020
I have a class which instances shall be presented as string if being used in the context of a string. To be precise, a attribute pointing to a string should be returned.
The class looks as the following and works so far:
class Test:
string = ""
def __init__(self, value=""):
self.string = value
def __repr__(self):
return str(self.string)
def __str__(self):
return self.__repr__()
def __add__(self, other):
return "%s%s" % (self.string, other)
def __radd__(self, other):
return "%s%s" % (other, self.string)
Unfortunately, I get a TypeError when I try doing something like this:
>>> "-".join([Test("test"), Test("test2")])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sequence item 0: expected string, instance found
Doing the same with a string, I get the expected result:
>>> "-".join(["test", "test"])
'test-test'
>>>
I'm pretty sure I'm just missing the correct operator overload function.
Could somebody how to achieve this with class instances in the list?
Thanks.
join requires elements to be strings, as documented here:
https://docs.python.org/3/library/stdtypes.html#str.join
You need to explicitly convert your objects to strings, e.g.:
"-".join(map(str, [Test("test"), Test("test2")]))