CODEWITHSUNDEEP
pythonif-statementpython-2.7return
Reno
Reno

Reputation: 1139

What is the difference between these two python functions?

I am having trouble understanding some of the short hand notation I am seeing in Python. Would someone be able to explain the difference between these two functions please? Thank you.

def test1():
   first = "David"
   last = "Smith"
   if first and last:
      print last


def test2():
   first = "David"
   last = "Smith"
   print first and last

Upvotes: 1

Views: 133

Answers (3)

pcurry
pcurry

Reputation: 1414

The difference between the functions test1() and test2() is that test1() explicitly prints the value of last so long as the result of the expression first and last evaluates as true, and test2() prints the result of the expression first and last. The string that is printed is the same because the result of the expression first and last is the value of last - but only because first evaluates as true.

In Python, if the left-hand side of an and expression evaluates as true, the result of the expression is right-hand side of that expression. Because of short-circuiting of boolean operators, if the left-hand-side of an and expression evaluates as false, the left-hand side of the expression is returned.

or also short-circuits in Python, returning the value of the leftmost part of the expression that determines the entire expression's truth value.

So, looking at a few more test functions:

def test3():
    first = ""
    last = "Smith"
    if first and last:
        print last

def test4():
    first = ""
    last = "Smith"
    print first and last

def test5():
    first = "David"
    last = "Smith"
    if first or last:
        print last

def test6():
    first = "David"
    last = "Smith"
    print first or last

def test7():
    first = "David"
    last = ""
    if first or last:
        print last

def test8():
    first = "David"
    last = ""
    print first or last

test3() will not print anything.
test4() will print "".

test5() will print "Smith".
test6() will print "David".

test7() will print "".
test8() will print "David".

Upvotes: 3

Robᵩ
Robᵩ

Reputation: 168626

What, you ask, is the difference between these two snippets?

if first and last:
  print last

and

print first and last

In the first case, either the code will print the value of last, or it won't.

In the second case, the code will print the value of first and last. If you are used to C, then you might think that the value of a and b is a boolean value of either True or False. But you'd be wrong.

a and b evaluates a; if a is truthy, the value of the expression is b. If a is falsely, the value of the expression is a:

"David" and "Smith" -> "Smith"
0 and "Smith" -> 0
1 and "Smith" -> "Smith" 
"David" and 0 -> 0
"David" and 1 -> 1

Generallly:

  • The first prints something sometimes, and doesn't print something other times.
    • The second prints something all of the time.
  • The first prints last, if it prints anything at all
    • The second prints either first or last, according to the truthiness of first.

Specifically, if first is ever "", then the 2nd example will print "" while the first won't print anything at all.

Upvotes: 0

mgilson
mgilson

Reputation: 309919

The first function always returns None (printing Smith) whereas the second one always returns "Smith"*

Quick digression into and:

the python and operator returns the first "falsy" value it encounters. If it doesn't encounter a "falsy" value, then it returns the last value (which is "true-y") This explains why:

"David" and "Smith"

always returns "Smith". Since both are non-empty strings, they are both "true-y" values.

"" and "Smith"

would return "" since it is a falsy value.

*The original function that OP posted actually looked like:

def test2():
   first = "David"
   last = "Smith"
   return first and last

Upvotes: 5

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