Unexpected behavior appending to a List of Dictionaries

[scidam]

You need to read about mutable and immutable objects in Python.

Dictionaries are mutable objects, so they could be changed in-place:

single_alien = {'color': 'grey', 'points': 8, 'speed': 'ultraslow'}

print(id(single_alien)) # print unique identifier of the object single_alien
# assume, that returned by `id` value is 12345

single_alien['color'] = 'green'
single_alien['points'] =  5
single_alien['speed'] = 'slow'

print(id(single_alien)) # lets print id again, after changes... 
# it is 12345! # So, the object was changed in-place, this is mutability...
aliens.append(single_alien)
 
single_alien['color'] = 'blue'
single_alien['points'] =  6
single_alien['speed'] = 'medium'

print(id(single_alien))
# it is still...  12345!
aliens.append(single_alien)

So, each time you appending single_alien to aliens, Python just duplicates references to the object single_alien;

for alien in aliens:
 print(id(alien))
 # will give the same values 

Each time you assigning a new dictionary to the single_alien,
internally, a new python object is created. You can check it with id.
ids will be different...

single_alien = {'color': 'green', 'points': 5, 'speed': 'slow'}
print(id(single_alien))
# eg. 123456
aliens.append(single_alien)

single_alien = {'color': 'blue', 'points': 6, 'speed': 'medium'}
print(id(single_alien))
# eg. 123234  123234!=123456, i.e. another object!
aliens.append(single_alien)