Introduction to Python

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In-Class Exercises, Session 9



PLEASE REFER to pythonreference.com for syntax to follow in coding these Exercises
 

FUNCTIONS: REVIEW
 
Ex. 9.1 Write a function that takes an argument and returns a value.

Function doubleit() takes one argument and returns its argument value doubled. 

# your def here
# you must not print inside the function
# you must not take input() inside the function
# you must not use 'y' inside the function



y = 5

dy = doubleit(y)     # calling 'doubleit()', expecting one return value
print(dy)            # 10
 
Ex. 9.2 Write another function that takes an argument and returns a value.

Write a function get_greeting() that takes one string argument (a name) and returns "Hello, " followed by the name and an exclamation point. 

# your def here
# you must not print inside the function
# you must not take input() inside the function
# you must not use 'x' or 'y' inside the function




x = 'World'
y = 'Guido'


# here, call get_greeting() with 'x' as argument
# make sure to expect a return value
# the return value should be 'Hello, World!'


# here, call get_greeting() with 'y' as argument
# make sure to expect a return value
# the return value should be 'Hello, Guido!'
 
Ex. 9.3 Write a function that takes two arguments and returns one value.

Define function multiplyem() that takes two numbers, multiplies them together, and returns the product. 

# your def here
# you must not print inside the function
# you must not take input() inside the function
# you must not use 'a' or 'b' inside the function


a = 5
b = 10



# here, call multiplyem() with 'a' and 'b' as arguments
# make sure to expect a return value
# the return value should be 50
 
Ex. 9.4 Write a function that takes one argument and returns two values.

Function mul_and_div takes an int argument and returns the value doubled and the value halved (/2) 

# your def here
# you must not print inside the function
# you must not take input() inside the function
# you must not use 'a' inside the function



a = 5

d, h = mul_and_div(a)

print(d)       # 10
print(h)       # 2.5
 
Ex. 9.5 Write a function that takes one positional (required) argument and one keyword (optional) argument.

Define function mul, which should take a required integer argument and an optional integer argument (keyword 'multiplier', with default 1. If the optional arg is not passed, it should default to 1 (so that the argument value is multiplied by 1 and thus returned unchanged). 

# your def here



a = 5

b = mul(a, multiplier=3)     # int, 15

c = mul(a)                   # int, 5
 
Ex. 9.6 Write a function that takes two positional (required) arguments and one keyword (optional) argument.

Define function get_field, which should take a required string argument, a required integer argument and an optional string argument (keyword 'sep', with default ','. The function takes a CSV line and an integer index. It splits the line on the separator and returns the field indicated by the integer -- see examples below. If the optional arg is not passed, it should default to ','. 

# your def here



v1 = get_field('hey:you:stop', 0, sep=':')   # 'hey'  (splits on the colon, then returns 1st item)

v2 = get_field('this,that,other', 1)         # 'that' (splits on the default comma, then returns 2nd item)
 
Ex. 9.7 Write a function that returns None (without explicitly returning it).

Write function greet(), which simply prints 'Hello, world!'. Printing x should show None. 

# make sure your function does not attempt to return anything



x = greet()
print(x)         # None
 
Ex. 9.8 Demonstration: remove a return statement and note the value.

The below function returns the sum of its arguments. First run the program and note the printed value, then remove the return statement to see the printed value. 

def get_sum(val1, val2):
    valsum = val1 + val2
    return valsum


r = get_sum(5, 10)
print(r)
 

MULTI-TARGET ASSIGNMENT
 
Ex. 9.9 Explore multi-target assignment errors.

Try removing a variable from either the left or right side of the =. What happens? 

a = 5
b = 10
c = 15

x, y, z = a, b, c
 

FUNCTIONS: VARIABLE SCOPES
 
Ex. 9.10 Explore function scoping.

Call this function, then attempt to print lvar after calling the function. Is lvar available outside the function? 

def do(arg):
    lvar = arg * 2
    return lvar


a = do(5)
print(a)
 
Ex. 9.11 Explore global variables inside functions.

Replace 2 with gvar inside the function, then call the function. Is gvar available inside the function? 

gvar = 2

def do(arg):
    lvar = arg * 2
    return lvar


a = do(5)
print(a)
 
Ex. 9.12 Identify local and global variables.

Identify the 3 local variables, 3 global variables and 2 builtin variables in the below code:

 
filename = '../pyku.txt'

def get_text(fname):
    fh = open(fname)
    text = fh.read()
    return text

txt = get_text(filename)
print(txt)
 

REVIEW: RAISE STATEMENT
 
Ex. 9.13 Raise an exception. Test to see if the user's input matches one of the values in the valid_choices list (use if uchoice not in valid_choices). If not, raise a ValueError exception with a descriptive error message. 
def validate_choice(uchoice):

    valid_choices = ['run', 'stop', 'rewind']

    if uchoice not in valid_choices:
        # your code here


validate_choice('run')           # should have no visible effect - function did not raise

validate_choice('fastforward')   # should raise ValueError with a message
 
Ex. 9.14 Detect object type and raise TypeError. Use the isinstance() function to see if the argument to this function is an integer. If it isn't, raise your own TypeError exception with a descriptive error message. 
def doubleit(arg):
    if not isinstance(arg, int):
        # your code here
    return arg * 2

x = doubleit('5')
 

LAB 1
 
Ex. 9.15 Multi-target assignment.

In one statement, assign the variables a, b and c to x, y and z respectively. 

a = 5
b = 10
c = 15

# your code here


print(x)            # 5
print(y)            # 10
print(z)            # 15
Expected Output: 
5
10
15
 
Ex. 9.16 Write a function that validates its argument.

Define function make_case(), which takes two arguments: text, a string to be modified, and case a string that must be either 'upper' or 'lower'. If case is not one of these, the function should raise a ValueError exception. Then based on the case argument, return the string uppercased or lowercased. 

# your function def here



greeting = 'Hello.'

x = make_case(greeting, 'upper')         # str, 'HELLO.'
print(x)

y = make_case(greeting, 'lower')         # str, 'hello.'
print(y)

z = make_case(greeting, 'middle')        # ValueError:  "middle" is not a valid argument:
                                    #              must be "upper" or "lower"
Expected Output: 
HELLO.
hello.
ValueError:  "middle" is not a valid argument:  must be "upper" or "lower"
 
Ex. 9.17 Write a function with an optional argument.

Modify the above function so that the 2nd argument is optional, with default 'upper'. Make sure it behaves as shown when called as shown below. 

# your function def here


greeting = 'Hello.'

a = make_case(greeting, case='lower')     # str, 'hello.'
print(a)

b = make_case(greeting, case='upper')     # str, 'HELLO.'
print(b)

c = make_case(greeting)                   # str, 'HELLO.'
print(c)

d = make_case(greeting, case='title')     # ValueError:  "title" is not a valid argument:
print(d)                                  #              must be "upper" or "lower"
Expected Output: 
hello.
HELLO.
HELLO.
ValueError:  "title" is not a valid argument:  must be "upper" or "lower"
 
Ex. 9.18 Write a function with both positional and optional arguments.

Write function temp_convert() that takes one required argument temp, a temperature value, and an optional argument scale=, which must be 'C' or 'F', and a default value of 'C'. If scale is not 'C' or 'F', the function raises a ValueError. Formula for F->C: (x - 32) * 5/9 Formula for C->F: (x * 9/5) + 32 Make sure the function behaves as shown below with the below arguments. 

# your function def here



c = temp_convert(212, scale='C')       # 100
print(c)

f = temp_convert(0, scale='F')         # 32
print(f)

cc = temp_convert(32)                  # 0
print(cc)

ff = temp_convert(-1, scale='Fahrenheit')   # ValueError:  "Fahrenheit" is not a
                                            # valid argument:  must be "C" or "F"
print(ff)
Expected Output: 
100
32
0
ValueError:  "Fahrenheit" is not a valid argument:  must be "C" or "F"
 
Ex. 9.19 Write a function that returns multiple values.

Define function name_split(), which takes a single string and attempts to split it on whitespace into 2 or 3 items. If there are more than 3 or fewer than 2 items, it raises a ValueError with a descriptive message. If there are three items, it returns them; if there are two items, it returns the first item, None, and the second item. Make sure the function return values are as shown below. 

# your function def here




first, second, third = name_split('George Patrick Flanahanahan')
print(first, second, third)      # 'George', 'Patrick', 'Flanahanahan'

fname, sname, tname = name_split('Aimee Vonda')
print(fname, sname, tname)       # 'Aimee', None, 'Vonda'

fn, sn, tn = name_split('Gimpy P. W. Simpson')
print(fn, sn, tn)                # ValueError:  must be 2 or 3 words in name
Expected Output: 
George Patrick Flanahanahan
Aimee None Vonda
ValueError:  must be 2 or 3 words in name
 
Ex. 9.20 Identify global and local variables.

Identify the global and local names in the below code. There are 3 local and 4 global names to be identified. 

a = 5

def do(c, d):
    e = c * d
    return e

b = 100

x = do(a, b)       # 500
print(x)
 

READING FROM MODULES
 
Ex. 9.21 Import a module and use its variables. Import module 'mymod' (located in the 'lib' directory) and print one of its variables. 
import sys

sys.path.append('../lib')    # temporary - adds mymod.py's directory
                             #             to your import path
                             #             (should not be used outside of class)
# your import and code here
 
Ex. 9.22 Import a module as a different name and use its variables. Perform the same as above but import the module as 'mm'. Again, print one of the varibles from mymod.py. 
import sys

sys.path.append('../lib')    # temporary - adds mymod.py's directory
                             #             to your import path
                             #             (should not be used outside of class)

# your import and code here
 
Ex. 9.23 Import a variable from a module into the program's namespace.

Perform the same as above but import a variable from 'mymod.py' directly into this program's namespace. You should be able to print this variable without prepending the name with mymod or mm. Again, print one of the varibles. 

import sys

sys.path.append('../lib')    # temporary - adds mymod.py's diretory
                             #             to your import path
                             #             (should not be used outside of class)

# your import and code here
 
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