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Solutions - Chapter 10
- 10-1: Learning Python
- 10-2: Learning C
- 10-3: Guest
- 10-4: Guest Book
- 10-5: Programming Poll
- 10-6: Addition
- 10-7: Addition Calculator
- 10-8: Cats and Dogs
- 10-9: Silent Cats and Dogs
- 10-11: Favorite Number
- 10-12: Favorite Number Remembered
- 10-13: Verify User
Back to solutions.
10-1: Learning Python
Open a blank file in your text editor and write a few lines summarizing what you’ve learned about Python so far. Start each line with the phrase In Python you can… Save the file as learning_python.txt in the same directory as your exercises fro mthis chapter. Write a program that reads the file and prints what you wrote three times. Print the contents once by reading in the entire file, once by looping over the file object, and once by storing the lines in a list and then working with them outside the with block.
learning_python.txt:
In Python you can store as much information as you want.
In Python you can connect pieces of information.
In Python you can model real-world situations.
learning_python.py:
filename = 'learning_python.txt'
print("--- Reading in the entire file:")
with open(filename) as f:
contents = f.read()
print(contents)
print("\n--- Looping over the lines:")
with open(filename) as f:
for line in f:
print(line.rstrip())
print("\n--- Storing the lines in a list:")
with open(filename) as f:
lines = f.readlines()
for line in lines:
print(line.rstrip())
Output:
--- Reading in the entire file: In Python you can store as much information as you want. In Python you can connect pieces of information. In Python you can model real-world situations. --- Looping over the lines: In Python you can store as much information as you want. In Python you can connect pieces of information. In Python you can model real-world situations. --- Storing the lines in a list: In Python you can store as much information as you want. In Python you can connect pieces of information. In Python you can model real-world situations.
10-2: Learning C
You can use the replace() method to replace any word in a string with a different word. Here’s a quick example showing how to replace 'dog' with 'cat' in a sentence:
>>> message = "I really like dogs."
>>> message.replace('dog', 'cat')
'I really like cats.'
Read in each line from the file you just created, learning_python.txt, and replace the word Python with the name of another language, such as C. Print each modified line to the screen.
filename = 'learning_python.txt'
with open(filename) as f:
lines = f.readlines()
for line in lines:
# Get rid of newline, then replace Python with C.
line = line.rstrip()
print(line.replace('Python', 'C'))
Output:
In C you can store as much information as you want.
In C you can connect pieces of information.
In C you can model real-world situations.
You can use rstrip() and replace() on the same line. This is called chaining methods. In the following code the newline is stripped from the end of the line and then Python is replaced by C. The output is identical to the code shown above.
filename = 'learning_python.txt'
with open(filename) as f:
lines = f.readlines()
for line in lines:
# Get rid of newline, then replace Python with C.
print(line.rstrip().replace('Python', 'C'))
10-3: Guest
Write a program that prompts the user for their name. When they respond, write their name to a file called guest.txt.
name = input("What's your name? ")
filename = 'guest.txt'
with open(filename, 'w') as f:
f.write(name)
Output:
What's your name? eric
guest.txt:
eric
10-4: Guest Book
Write a while loop that prompts users for their name. When they enter their name, print a greeting to the screen and add a line recording their visit in a file called guest_book.txt. Make sure each entry appears on a new line in the file.
filename = 'guest_book.txt'
print("Enter 'quit' when you are finished.")
while True:
name = input("\nWhat's your name? ")
if name == 'quit':
break
else:
with open(filename, 'a') as f:
f.write(name + "\n")
print("Hi " + name + ", you've been added to the guest book.")
Output:
Enter 'quit' when you are finished. What's your name? eric Hi eric, you've been added to the guest book. What's your name? willie Hi willie, you've been added to the guest book. What's your name? ever Hi ever, you've been added to the guest book. What's your name? erin Hi erin, you've been added to the guest book. What's your name? quit
guest_book.txt:
eric
willie
ever
erin
10-5: Programming Poll
Write a while loop that asks people why they like programming. Each time someone enters a reason, add their reason to a file that stores all the responses.
filename = 'programming_poll.txt'
responses = []
while True:
response = input("\nWhy do you like programming? ")
responses.append(response)
continue_poll = input("Would you like to let someone else respond? (y/n) ")
if continue_poll != 'y':
break
with open(filename, 'a') as f:
for response in responses:
f.write(response + "\n")
Output:
Why do you like programming? Programmers can build almost anything they can imagine. Would you like to let someone else respond? (y/n) y Why do you like programming? It's really fun, and really satisfying. Would you like to let someone else respond? (y/n) y Why do you like programming? It just never gets old. Would you like to let someone else respond? (y/n) n
programming_poll.txt:
Programmers can build almost anything they can imagine.
It's really fun, and really satisfying.
It just never gets old.
10-6: Addition
One common problem when prompting for numerical input occurs when people provide text instead of numbers. When you try to convert the input to an int, you’ll get a ValueError. Write a program that prompts for two numbers. Add them together and print the result. Catch the TypeError if either input value is not a number, and print a friendly error message. Test your program by entering two numbers and then by entering some text instead of a number.
try:
x = input("Give me a number: ")
x = int(x)
y = input("Give me another number: ")
y = int(y)
except ValueError:
print("Sorry, I really needed a number.")
else:
sum = x + y
print("The sum of " + str(x) + " and " + str(y) + " is " + str(sum) + ".")
Output with two integers:
Give me a number: 23 Give me another number: 47 The sum of 23 and 47 is 70.
Output with non-numerical input:
Give me a number: 23 Give me another number: fred Sorry, I really needed a number.
10-7: Addition Calculator
Wrap your code from Exercise 10-6 in a while loop so the user can continue entering numbers even if they make a mistake and enter text instead of a number.
print("Enter 'q' at any time to quit.\n")
while True:
try:
x = input("\nGive me a number: ")
if x == 'q':
break
x = int(x)
y = input("Give me another number: ")
if y == 'q':
break
y = int(y)
except ValueError:
print("Sorry, I really needed a number.")
else:
sum = x + y
print("The sum of " + str(x) + " and " + str(y) + " is " + str(sum) + ".")
Output:
Enter 'q' at any time to quit. Give me a number: 23 Give me another number: 47 The sum of 23 and 47 is 70. Give me a number: three Sorry, I really needed a number. Give me a number: 3 Give me another number: five Sorry, I really needed a number. Give me a number: -12 Give me another number: 20 The sum of -12 and 20 is 8. Give me a number: q
10-8: Cats and Dogs
Make two files, cats.txt and dogs.txt. Store at least three names of cats in the first file and three names of dogs in the second file. Write a program that tries to read these files and print the contents of the file to the screen. Wrap your code ina try-except block to catch the FileNotFound error, and print a friendly message if a file is missing. Move one of the files to a different location on your system, and make sure the code in the except block executes properly.
cats.txt:
henry
clarence
mildred
dogs.txt:
willie
annahootz
summit
cats_and_dogs.py:
filenames = ['cats.txt', 'dogs.txt']
for filename in filenames:
print("\nReading file: " + filename)
try:
with open(filename) as f:
contents = f.read()
print(contents)
except FileNotFoundError:
print(" Sorry, I can't find that file.")
Output with both files:
Reading file: cats.txt
henry
clarence
mildred
Reading file: dogs.txt
willie
annahootz
summit
Output after moving cats.txt:
Reading file: cats.txt
Sorry, I can't find that file.
Reading file: dogs.txt
willie
annahootz
summit
10-9: Silent Cats and Dogs
Modify your except block in Exercise 10-8 to fail silently if either file is missing.
filenames = ['cats.txt', 'dogs.txt']
for filename in filenames:
try:
with open(filename) as f:
contents = f.read()
except FileNotFoundError:
pass
else:
print("\nReading file: " + filename)
print(contents)
Output when both files exist:
Reading file: cats.txt
henry
clarence
mildred
Reading file: dogs.txt
willie
annahootz
summit
Output when cats.txt has been moved:
Reading file: dogs.txt
willie
annahootz
summit
10-11: Favorite Number
Write a program that prompts for the user’s favorite number. Use json.dump() to store this number in a file. Write a separate program that reads in this value and prints the message, “I know your favorite number! It’s _____.”
favorite_number_write.py:
import json
number = input("What's your favorite number? ")
with open('favorite_number.json', 'w') as f:
json.dump(number, f)
print("Thanks! I'll remember that.")
Output:
What's your favorite number? 42 Thanks! I'll remember that.
favorite_number_read.py:
import json
with open('favorite_number.json') as f:
number = json.load(f)
print("I know your favorite number! It's " + str(number) + ".")
Output:
I know your favorite number! It's 42.
10-12: Favorite Number Remembered
Combine the two programs from Exercise 10-11 into one file. If the number is already stored, report the favorite number to the user. If not, prompt for the user’s favorite number and store it in a file. Run the program twice to see that it works.
import json
try:
with open('favorite_number.json') as f:
number = json.load(f)
except FileNotFoundError:
number = input("What's your favorite number? ")
with open('favorite_number.json', 'w') as f:
json.dump(number, f)
print("Thanks, I'll remember that.")
else:
print("I know your favorite number! It's " + str(number) + ".")
Output, first run:
What's your favorite number? 42 Thanks, I'll remember that.
Output, second run:
I know your favorite number! It's 42.
10-13: Verify User
The final listing for remember_me.py assumes either that the user has already entered their username or that the program is running for the first time. We should modify it in case the current user is not the person who last used the program.
Before printing a welcome back message in greet_user(), ask the user if this is the correct username. If it’s not, call get_new_username() to get the correct username.
import json
def get_stored_username():
"""Get stored username if available."""
filename = 'username.json'
try:
with open(filename) as f_obj:
username = json.load(f_obj)
except FileNotFoundError:
return None
else:
return username
def get_new_username():
"""Prompt for a new username."""
username = input("What is your name? ")
filename = 'username.json'
with open(filename, 'w') as f_obj:
json.dump(username, f_obj)
return username
def greet_user():
"""Greet the user by name."""
username = get_stored_username()
if username:
correct = input("Are you " + username + "? (y/n) ")
if correct == 'y':
print("Welcome back, " + username + "!")
else:
username = get_new_username()
print("We'll remember you when you come back, " + username + "!")
else:
username = get_new_username()
print("We'll remember you when you come back, " + username + "!")
greet_user()
Output:
> python verify_user.py What is your name? eric We'll remember you when you come back, eric! > python verify_user.py Are you eric? (y/n) y Welcome back, eric! > python verify_user.py Are you eric? (y/n) n What is your name? ever We'll remember you when you come back, ever! > python verify_user.py Are you ever? (y/n) y Welcome back, ever!
You might notice the identical else blocks in this version of greet_user(). One way to clean this function up is to use an empty return statement. An empty return statement tells Python to leave the function without running any more code in the function.
Here’s a cleaner version of greet_user():
def greet_user():
"""Greet the user by name."""
username = get_stored_username()
if username:
correct = input("Are you " + username + "? (y/n) ")
if correct == 'y':
print("Welcome back, " + username + "!")
return
# We got a username, but it's not correct.
# Let's prompt for a new username.
username = get_new_username()
print("We'll remember you when you come back, " + username + "!")
The return statement means the code in the function stops running after printing the welcome back message. When the username doesn’t exist, or the username is incorrect, the return statement is never reached. The second part of the function will only run when the if statements fail, so we don’t need an else block. Now the function prompts for a new username when either if statement fails.
The only thing left to address is the nested if statements. This can be cleaned up by moving the code that checks whether the username is correct to a separate function. If you’re enjoying this exercise, you might try making a new function called check_username() and see if you can remove the nested if statement from greet_user().