What do the parenthesis following a lambda expression mean?

Question

I am new to Python and I am wondering why this doesn't work for lambda:

for person in people:
    print(lambda person:person.split()[0] + ' ' + person.split()[-1])

results:

<function <lambda> at 0x7f9d04332ed8>
<function <lambda> at 0x7f9d04332ed8>
<function <lambda> at 0x7f9d04332ed8>
<function <lambda> at 0x7f9d04332ed8>

But instead, the solution given was

for person in people:
    print((lambda person:person.split()[0] + ' ' + person.split()[-1])(person))

Results:

Dr. Brooks
Dr. Collins-Thompson
Dr. Vydiswaran
Dr. Romero

What does it mean to add (person) behind the lambda expression? And what does the first result mean?

Answer 1

A lambda expression defines an anonymous function.
Like other functions, it doesn't do anything unless you call it.

I suspect that you might be confusing yourself by naming the lambda parameter the same as the loop variable - they are entirely different things.
With unique naming, your code becomes

for person in people:
    print(lambda p:p.split()[0] + ' ' + p.split()[-1])

for person in people:
    print((lambda p:p.split()[0] + ' ' + p.split()[-1])(person))

which makes the difference clearer.
Note that the first version doesn't use the loop variable at all.

Even clearer is a version with a named function:

def the_function(p):
    return p.split()[0] + ' ' + p.split()[-1]

# First attempt
for person in people:
    print(the_function)

# Second attempt
for person in people:
    print(the_function(person))

Answer 2

A lambda is a function. Yours might be written like this:

lambda person:person.split()[0] + ' ' + person.split()[-1]

But it's the same as this:

def myLambda(person):
  return person.split()[0] + ' ' + person.split()[-1]

And of course both need to be called. To call the function one would use its name, followed by parenthesis: myLambda() but you didn't allocate a name to the lambda function. Instead, by wrapping it in parenthesis, we get a function. You can then put parenthesis on the end to call it:

# ------------ lambda function is in parenthesis -----------##-and then called with the `person` variable
(lambda person:person.split()[0] + ' ' + person.split() [-1])(person)