Can't get a for loop to work as list comprehension

Question

This for loop works fine but not as a list comprehension. The comprehension creates empty elements for each iteration. Can someone explain what is wrong and how to fix it?

def GetAsciiList():
    num_list = []
    for num in range(32, 42, 1):
        num_list.append((str(num), chr(num)+" "))
    return num_list

def GetAsciiListLC():
    num_list = []
    num_list = [(num_list.append((str(num), chr(num)+" "))) for num in range(32, 42, 1)]
    return num_list

print GetAsciiList()
print GetAsciiListLC()

console output:

[('32', '  '), ('33', '! '), ('34', '" '), ('35', '# '), ('36', '$ '), ('37', '% '), ('38', '& '), ('39', "' "), ('40', '( '), ('41', ') ')]
[None, None, None, None, None, None, None, None, None, None]

Can you edit your question to format your Python code? Python is whitespace-sensitive and it's hard to know how to correctly edit this question. — NicholasM, Feb 01 '20 at 19:39
List comprehensions resolve to a list. So you probably just want `return [(str(num), chr(num)+" ") for num in range(32, 42, 1)]`. If you append in the list comprehension, the result of the appending, `None` will be the value for each item in the list, as you are seeing. — audiodude, Feb 01 '20 at 19:42

score 2 · Accepted Answer · answered Feb 01 '20 at 19:41

Consider this line in GetAsciiListLC():

num_list = [(num_list.append((str(num), chr(num)+" "))) for num in range(32, 42, 1)]

num_list.append((str(num), chr(num)+" "))) mutates the list and returns None.

I think what you want is this:

def GetAsciiListLC():
    """ Return list of 2-tuples containing numbers and ASCII equivalents, both as strings. """
    return [(str(num), chr(num) + " ") for num in range(32, 42)]

See this question for the rationale of returning None from the list.append() method: Why does append() always return None in Python?

Can't get a for loop to work as list comprehension

1 Answers1