Starter Files

Download Inside the archive, you will find starter files for the questions in this lab, along with a copy of the OK autograder.


By the end of this lab, you should have submitted the lab with python3 ok --submit. You may submit more than once before the deadline; only the final submission will be graded. Check that you have successfully submitted your code on

  • To receive full credit for this lab, all questions must be attempted.

When you are ready to submit, run ok with the --submit option:

python3 ok --submit

After submitting, ok will display a submission URL, with which you can view your submission on


Lambda expressions are one-line functions that specify two things: the parameters and the return value.

lambda <parameters>: <return value>

While both lambda and def statements are related to functions, there are some differences.

lambda def
Type lambda is an expression def is a statement
Description Evaluating a lambda expression does not create or modify any variables. Lambda expressions just create new function objects. Executing a def statement will create a new function object and bind it to a variable in the current environment.
lambda x: x * x
def square(x):
    return x * x

A lambda expression by itself is not very interesting. As with any objects such as numbers, booleans, strings, we usually:

  • assign lambda to variables (foo = lambda x: x)
  • pass them in to other functions (bar(lambda x: x))
  • return them as the results of other functions (return lambda x: x)
  • return them as the results of other lambdas (lambda x: lambda y: x + y)

In the final example above, the outer lambda (lambda x) takes in a value x, and it returns another lambda (lambda y) that takes an argument y and returns x+y.

Environment Diagrams

Environment diagrams are one of the best learning tools for understanding lambda expressions because you're able to keep track of all the different names, function objects, and arguments to functions. We highly recommend drawing environment diagrams or using Python tutor if you get stuck doing the WWPD problems below. For examples of what environment diagrams should look like, try running some code in Python tutor. Here are the rules:


Note: As we saw in the lambda expression section above, lambda functions have no intrinsic name. When drawing lambda functions in environment diagrams, they are labeled with the name lambda or with the lowercase Greek letter λ. This can get confusing when there are multiple lambda functions in an environment diagram, so you can distinguish them by numbering them or by writing the line number on which they were defined.

  1. Draw the lambda function object and label it with λ, its formal parameters, and its parent frame. A function's parent frame is the frame in which the function was defined.

This is the only step. We are including this section to emphasize the fact that the difference between lambda expressions and def statements is that lambda expressions do not create any new bindings in the environment.


Question 1: WWPD: Lambda the Free

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q lambda -u

For all WWPD questions, type Function if you believe the answer is <function...>, Error if it errors, and Nothing if nothing is displayed. As a reminder, the following two lines of code will not display anything in the Python interpreter when executed:

>>> x = None
>>> x
>>> lambda x: x  # A lambda expression with one parameter x
<function <lambda> at ...>
>>> a = lambda x: x # Assigning the lambda function to the name a >>> a(5)
>>> (lambda: 3)() # Using a lambda expression as an operator in a call exp.
>>> b = lambda x: lambda: x # Lambdas can return other lambdas! >>> c = b(88) >>> c
<function <lambda> at ...
>>> c()
>>> d = lambda f: f(4) # They can have functions as arguments as well. >>> def square(x): ... return x * x >>> d(square)
>>> z = 3
>>> e = lambda x: lambda y: lambda: x + y + z
>>> e(0)(1)()
>>> f = lambda z: x + z >>> f(3)
NameError: name 'x' is not defined
>>> higher_order_lambda = lambda f: lambda x: f(x)
>>> g = lambda x: x * x
>>> higher_order_lambda(2)(g)  # Which argument belongs to which function call?
>>> higher_order_lambda(g)(2)
>>> call_thrice = lambda f: lambda x: f(f(f(x))) >>> call_thrice(lambda y: y + 1)(0)
>>> print_lambda = lambda z: print(z) # When is the return expression of a lambda expression executed? >>> print_lambda
>>> one_thousand = print_lambda(1000)
>>> one_thousand
# print_lambda returned None, so nothing gets displayed


Question 2: Mul_by_num

Using a lambda expression, complete the mul_by_num function. This function should take an argument num and return a one argument function that multiplies any value passed to it by num. Its body must be one line long:

def mul_by_num(num):
    Returns a function that takes one argument and returns num
    times that argument.
    >>> x = mul_by_num(5)
    >>> y = mul_by_num(2)
    >>> x(3)
    >>> y(-4)
"*** YOUR CODE HERE ***"
return lambda num2: num * num2

Use OK to test your code:

python3 ok -q mul_by_num

Question 3: Compose

Write a function that takes in 2 single-argument functions, f and g, and returns another lambda function that takes in a single argument x. The returned function should return the output of applying f(g(x)).

Hint: The staff solution is only 1 line!

def compose(f, g):
    """Write a function that takes in 2 single-argument functions, f and g, and returns another lambda function 
    that takes in a single argument x. The returned function should return the output of applying f(g(x)). 
    Hint: The staff solution is only 1 line!

    Return the composition function which given x, computes f(g(x)). 

    >>> add_two = lambda x: x + 2  		# adds 2 to x
    >>> square = lambda x: x ** 2 		# squares x
    >>> a = compose(square, add_two) 	# (x + 2 ) ^ 2
    >>> a(5) 
    >>> mul_ten = lambda x: x * 10 		# multiplies 10 with x
    >>> b = compose(mul_ten, a) 		# ((x + 2 ) ^ 2) * 10
    >>> b(5)
    >>> b(2)
"*** YOUR CODE HERE ***"
return lambda x: f(g(x))

Use OK to test your code:

python3 ok -q compose

Question 4: Counter

Implement the function counter which takes in a string of words message, and returns a dictionary where each key is a word in the message, and each value is the number of times that word is present in the original string.

def counter(message):
    """ Returns a dictionary of each word in message mapped
    to the number of times it appears in the input string.

    >>> x = counter('to be or not to be')
    >>> x['to']
    >>> x['be']
    >>> x['not']
    >>> y = counter('run forrest run')
    >>> y['run']
    >>> y['forrest']
    word_list = message.split()
"*** YOUR CODE HERE ***"
result_dict = {} for word in word_list: if word in result_dict: result_dict[word] += 1 else: result_dict[word] = 1 return result_dict

Use OK to test your code:

python3 ok -q counter

Question 5: Build the Full Rosters

Implement the function common_players. The common_players function takes in a roster dictionary and identifies which keys share the same values. The function returns a new dictionary of this structure:

  • Keys: The values from the roster dictionary
  • Values: A list of keys from roster that share that same value.
def common_players(roster):
    """Returns a dictionary containing values along with a corresponding
    list of keys that had that value from the original dictionary.
    >>> full_roster = {"bob": "Team A", "barnum": "Team B", "beatrice": "Team C", "bernice": "Team B", "ben": "Team D", "belle": "Team A", "bill": "Team B", "bernie": "Team B", "baxter": "Team A"}
    >>> player_dict = common_players(full_roster)
    >>> dict(sorted(player_dict.items()))
    {'Team A': ['bob', 'belle', 'baxter'], 'Team B': ['barnum', 'bernice', 'bill', 'bernie'], 'Team C': ['beatrice'], 'Team D': ['ben']}
"*** YOUR CODE HERE ***"
result_dict = {} for player in roster: team = roster[player] if team in result_dict: result_dict[team] += [player] else: result_dict[team] = [player] return result_dict

Use OK to test your code:

python3 ok -q common_players


Make sure to submit this assignment by running:

python3 ok --submit

Optional: Environment Diagram Practice

There is no submission for this component. However, we still encourage you to do these problems on paper to develop familiarity with Environment Diagrams, which will appear on the exam.

Question 6: Lambda the Environment Diagram

Try drawing an environment diagram for the following code and predict what Python will output.

You do not need to submit or unlock this question through Ok. Instead, you can check your work with the Online Python Tutor, but try drawing it yourself first!

>>> a = lambda x: x * 2 + 1
>>> def b(b, x):
...     return b(x + a(x))
>>> x = 3
>>> b(a, x)
21 # Interactive solution: