Question

I can’t understand why the code below works like this. (in Python3x)

>>>f = lambda: print('Hello') or print('Hello again')

>>>f()

Hello

Hello again

and also can’t understand like this.

>>>f = lambda: print('Hello') and print('Hello again')

>>>f()

Hello

For me, the first lambda function seems to print the word ‘Hello’ or the word ‘Hello again’ but it print both ‘Hello’ and ‘Hello again’.

The second function seems to print the word ‘Hello’ and ‘Hello again’ but it print only ‘Hello’.

Can anybody explain what is going on in this lambda function?

Thank you for your kind help!

Answer 1

print returns None (which is Falseish), so python has to evaluate the other operand of or, but not and, to get the answer.

• return print('Hello') or print('Hello again')
• Hello is printed, print returns None
• return None or print('Hello again')
• or returns True if any of the operands is True. If the first one is True, there’s no need to evaluate the second one. This isn’t the case
• Hello again is printed
• return None or None
• now we are certain False should be returned.

 

• return print('Hello') and print('Hello again')
• Hello is printed, print returns None
• return None and print('Hello again')
• and returns True if both operands are True. If the first one is False, there’s no need to evaluate the second one.
• return False