Python string.join(list) with an array of objects

python

object-transformation

string-methods

best-practices

byAlex Kataev·Mar 3, 2025

To concatenate an object array into one string in Python, a transformation of objects to strings is necessary. Use list comprehension to do this efficiently:

# Drum roll, please 🥁...
joined_string = ' '.join([str(obj) for obj in objects_array])

Or, the map function can offer an equally concise solution:

# And now for our next trick...
joined_string = ' '.join(map(str, objects_array))

In both cases, __str__ of each object is invoked, and the objects are tied together into one single, space-separated string.

Object transformation 101

Every object is equipped with a __str__ method that defines how it appears as a string. This representation is called upon whenever you use str(obj). If not specified within your objects, Python will default to using __repr__:

class MyObject:
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return f'Custom({self.value})'  # Ha, I pull the strings now!

Calling str() on an object of MyObject now gets you Custom(value).

Play nice with built-ins

Avoid the naming temptation of list as it overshadows Python's built-in list type. Choose clear and descriptive names instead:

# Not all heroes wear capes; some choose descriptive variable names.
my_objects = [MyObject(1), MyObject(2)]
joined_string = ' '.join(str(obj) for obj in my_objects)

Custom strings for the connoisseur

For users desiring more finesse, a custom subclass of str can override the join method to handle non-string objects:

# When str isn't enough, you make your own!
class MyStr(str):
    def join(self, iterable):
        strings = (str(item) for item in iterable)
        return super().join(strings)

By creating an instance of MyStr, you can join objects fluently:

# Yes, alongside pet rocks, I also collect custom strings.
joiner = MyStr(' - ')
joined_string = joiner.join(my_objects)

Str and repr: a harmonious duet

It is often sensible to make __repr__ align with __str__ for consistent outputs:

class Object:
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return f'Object({self.value})'
    __repr__ = __str__  # Duet mode engaged.