How to make print call the __str__ method of Python objects inside a list?

You can use a list comprehension to generate a new list with each item str()’d automatically:

print([str(item) for item in mylist])

Two easy things you can do, use the map function or use a comprehension.

But that gets you a list of strings, not a string. So you also have to join the strings together.

s= ",".join( map( str, myList ) )

or

s= ",".join( [ str(element) for element in myList ] )

Then you can print this composite string object.

print 'my list is %s'%( s )

Something like this?

a = [1, 2 ,3]
[str(x) for x in a]
# ['1', '2', '3']

Depending on what you want to use that output for, perhaps __repr__ might be more appropriate:

import unittest

class A(object):
    def __init__(self, val):
        self.val = val

    def __repr__(self):
        return repr(self.val)

class Test(unittest.TestCase):
    def testMain(self):
        l = [A('a'), A('b')]
        self.assertEqual(repr(l), "['a', 'b']")

if __name__ == '__main__':
    unittest.main()

I agree with the previous answer about using list comprehensions to do this, but you could certainly hide that behind a function, if that’s what floats your boat.

def is_list(value):
    if type(value) in (list, tuple): return True
    return False

def list_str(value):
    if not is_list(value): return str(value)
    return [list_str(v) for v in value]

Just for fun, I made list_str() recursively str() everything contained in the list.

The output you’re getting is just the object’s module name, class name, and then the memory address in hexadecimal as the the __repr__ function is not overridden.

__str__ is used for the string representation of an object when using print. But since you are printing a list of objects, and not iterating over the list to call the str method for each item it prints out the objects representation.

To have the __str__ function invoked you’d need to do something like this:

'my list is %s' % [str(x) for x in myList]

If you override the __repr__ function you can use the print method like you were before:

class cell:
    def __init__(self, id):
        self.id = id
    def __str__(self):
        return str(self.id) # Or whatever
    def __repr__(self):
        return str(self) # function invoked when you try and print the whole list.

myList = [cell(1), cell(2), cell(3)]
'my list is %s' % myList

Then you’ll get “my list is [1, 2, 3]” as your output.

This should suffice.

When printing lists as well as other container classes, the contained elements will be printed using __repr__, because __repr__ is meant to be used for internal object representation.
If we call: help(object.__repr__) it will tell us:

Help on wrapper_descriptor:

__repr__(self, /)
    Return repr(self).

And if we call help(repr) it will output:

Help on built-in function repr in module builtins:

repr(obj, /)
    Return the canonical string representation of the object.

    For many object types, including most builtins, eval(repr(obj)) == obj.

If __str__ is implemented for an object and __repr__ is not repr(obj) will output the default output, just like print(obj) when non of these are implemented.

So the only way is to implement __repr__ for your class. One possible way to do that is this:

class C:           
    def __str__(self):
        return str(f"{self.__class__.__name__} class str ")

C.__repr__=C.__str__       
ci = C()    


print(ci)       #C class str 
print(str(ci))  #C class str 
print(repr(ci)) #C class str