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Why don't format specifiers work with lists, dictionaries and other objects?

When I want to print a number or a string, I can use [*f-strings*](https://docs.python.org/3/reference/lexical_analysis.html#f-strings) (Python >= 3.6) ou [`str.format`][1], and I can use just the variable between braces, or use [format specifiers](https://docs.python.org/3/library/string.html#formatspec). Ex:

```python
num, text = 10, 'abc'

# passing just the variables
print(f'{num} {text}')
# or
#print('{} {}'.format(num, text))

# using format specifiers
# number left-aligned, with 6 spaces, text right-aligned with 10 spaces
print(f'{num:<6} {text:>10}')
# or
#print('{:<6} {:>12}'.format(num, text))
```

Output:

```none
10 abc
10            abc
```

But if I do the same with lists or dictionaries, only the first option works:

```python
mylist = [1, 2]
dic = {'a': 1}
# this works
print(f'{mylist} {dic}')
# or
#print('{} {}'.format(mylist, dic))

# this doesn't work
print(f'{mylist:<10} {dic:>15}')
# or
#print('{:<10} {:>15}'.format(mylist, dic))
```

The first `print` outputs:

```none
[1, 2] {'a': 1}
```

But the second `print` (with the format specifiers `<10` and `>15`) gives this error:

>     TypeError: unsupported format string passed to list.__format__

---
If I try the same thing with an instance of a class that I created, the same thing occurs. Ex:

```python
class Test:
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return f'Test({self.value})'

t = Test(42)
print(f't={t}') # this works
print(f'{t:>10}') # this doesn't work
```

The first `print` outputs:

```none
t=Test(42)
```

The second `print` gives an error:

>     TypeError: unsupported format string passed to Test.__format__

---

My question is **not** about how to fix it (*I could just convert the list/dictionary/object to string, either by using [`str`](https://docs.python.org/3/library/functions.html#func-str), or by iterating through its elements or atributes and manually building the string, etc*).

What I want to know is **why this happens**. Why it's not possible to use format specifiers with lists, dictionaries and instances of my own classes, but if I pass them without any specifiers, it works? Are there any internal details about those types, that make them behave differently to numbers and strings, when those are formatted?


  [1]: https://docs.python.org/3/library/stdtypes.html#str.format