Unexpect string format behaviour [duplicate]

This question already has an answer here:

• Printing boolean values True/False with the format() method in Python 1 answer

I'm seeing some unexpected behaviour with how str.format() handles bool types when specifying alignment. Here's the example:

>>> '{}'.format(bool(1))
True

>>> '{:<}'.format(bool(1))
1

Notice -- when I specified alignment, format switched to displaying an integer instead of True (I wanted to display the bool, not an integer). I can force the desired output by casting the bool to string thusly:

>>> '{:<}'.format(str(bool(1)))
True

My question is -- can anyone explain this edge case in str.format() ?