Typing string values in Python code is fairly straightforward: They begin and end with a single quote. But then how can you use a quote inside a string? Typing 'That is Alice's cat.' won’t work, because Python thinks the string ends after Alice, and the rest (s cat.') is invalid Python code. Fortunately, there are multiple ways to type strings.

>>> spam = "That is Alice's cat."

>>> spam = 'Say hi to Bob\'s mother.'

>>> print("Hello there!\nHow are you?\nI\'m doing fine.")
Hello there!
How are you?
I'm doing fine.

>>> print(r'That is Carol\'s cat.')
That is Carol\'s cat.

print('''Dear Alice,

Eve's cat has been arrested for catnapping, cat burglary, and extortion.

Sincerely,
Bob''')

Dear Alice,

Eve's cat has been arrested for catnapping, cat burglary, and extortion.

Sincerely,
Bob

print('Dear Alice,\n\nEve\'s cat has been arrested for catnapping, cat
burglary, and extortion.\n\nSincerely,\nBob')

"""This is a test Python program.
Written by Al Sweigart al@inventwithpython.com

This program was designed for Python 3, not Python 2.
"""

def spam():
    """This is a multiline comment to help
    explain what the spam() function does."""
    print('Hello!')

Strings use indexes and slices the same way lists do. You can think of the string 'Hello world!' as a list and each character in the string as an item with a corresponding index.

'   H   e   l   l   o       w   o   r   l   d    !   '
    0   1   2   3   4   5   6   7   8   9   10   11

The space and exclamation point are included in the character count, so 'Hello world!' is 12 characters long, from H at index 0 to ! at index 11.

Enter the following into the interactive shell:

>>> spam = 'Hello world!'
>>> spam[0]
'H'
>>> spam[4]
'o'
>>> spam[-1]
'!'
>>> spam[0:5]
'Hello'
>>> spam[:5]
'Hello'
>>> spam[6:]
'world!'

If you specify an index, you’ll get the character at that position in the string. If you specify a range from one index to another, the starting index is included and the ending index is not. That’s why, if spam is 'Hello world!', spam[0:5] is 'Hello'. The substring you get from spam[0:5] will include everything from spam[0] to spam[4], leaving out the space at index 5.

Note that slicing a string does not modify the original string. You can capture a slice from one variable in a separate variable. Try typing the following into the interactive shell:

>>> spam = 'Hello world!'
>>> fizz = spam[0:5]
>>> fizz
'Hello'

By slicing and storing the resulting substring in another variable, you can have both the whole string and the substring handy for quick, easy access.

The in and not in operators can be used with strings just like with list values. An expression with two strings joined using in or not in will evaluate to a Boolean True or False. Enter the following into the interactive shell:

>>> 'Hello' in 'Hello World'
True
>>> 'Hello' in 'Hello'
True
>>> 'HELLO' in 'Hello World'
False
>>> '' in 'spam'
True
>>> 'cats' not in 'cats and dogs'
False

These expressions test whether the first string (the exact string, case sensitive) can be found within the second string.

The upper() and lower() string methods return a new string where all the letters in the original string have been converted to uppercase or lower-case, respectively. Nonletter characters in the string remain unchanged. Enter the following into the interactive shell:

>>> spam = 'Hello world!'
>>> spam = spam.upper()
>>> spam
'HELLO WORLD!'
>>> spam = spam.lower()
>>> spam
'hello world!'

Note that these methods do not change the string itself but return new string values. If you want to change the original string, you have to call upper() or lower() on the string and then assign the new string to the variable where the original was stored. This is why you must use spam = spam.upper() to change the string in spam instead of simply spam.upper(). (This is just like if a variable eggs contains the value 10. Writing eggs + 3 does not change the value of eggs, but eggs = eggs + 3 does.)

The upper() and lower() methods are helpful if you need to make a case-insensitive comparison. The strings 'great' and 'GREat' are not equal to each other. But in the following small program, it does not matter whether the user types Great, GREAT, or grEAT, because the string is first converted to lowercase.

print('How are you?')
feeling = input()
if feeling.lower() == 'great':
    print('I feel great too.')
else:
    print('I hope the rest of your day is good.')

When you run this program, the question is displayed, and entering a variation on great, such as GREat, will still give the output I feel great too. Adding code to your program to handle variations or mistakes in user input, such as inconsistent capitalization, will make your programs easier to use and less likely to fail.

How are you?
GREat
I feel great too.

The isupper() and islower() methods will return a Boolean True value if the string has at least one letter and all the letters are uppercase or lowercase, respectively. Otherwise, the method returns False. Enter the following into the interactive shell, and notice what each method call returns:

>>> spam = 'Hello world!'
>>> spam.islower()
False
>>> spam.isupper()
False
>>> 'HELLO'.isupper()
True
>>> 'abc12345'.islower()
True
>>> '12345'.islower()
False
>>> '12345'.isupper()
False

Since the upper() and lower() string methods themselves return strings, you can call string methods on those returned string values as well. Expressions that do this will look like a chain of method calls. Enter the following into the interactive shell:

>>> 'Hello'.upper()
'HELLO'
>>> 'Hello'.upper().lower()
'hello'
>>> 'Hello'.upper().lower().upper()
'HELLO'
>>> 'HELLO'.lower()
'hello'
>>> 'HELLO'.lower().islower()
True

Along with islower() and isupper(), there are several string methods that have names beginning with the word is. These methods return a Boolean value that describes the nature of the string. Here are some common isX string methods:

Enter the following into the interactive shell:

>>> 'hello'.isalpha()
True
>>> 'hello123'.isalpha()
False
>>> 'hello123'.isalnum()
True
>>> 'hello'.isalnum()
True
>>> '123'.isdecimal()
True
>>> '    '.isspace()
True
>>> 'This Is Title Case'.istitle()
True
>>> 'This Is Title Case 123'.istitle()
True
>>> 'This Is not Title Case'.istitle()
False
>>> 'This Is NOT Title Case Either'.istitle()
False

The isX string methods are helpful when you need to validate user input. For example, the following program repeatedly asks users for their age and a password until they provide valid input. Open a new file editor window and enter this program, saving it as validateInput.py:

while True:
    print('Enter your age:')
    age = input()
    if age.isdecimal():
        break
    print('Please enter a number for your age.')

while True:
    print('Select a new password (letters and numbers only):')
    password = input()
    if password.isalnum():
        break
    print('Passwords can only have letters and numbers.')

In the first while loop, we ask the user for their age and store their input in age. If age is a valid (decimal) value, we break out of this first while loop and move on to the second, which asks for a password. Otherwise, we inform the user that they need to enter a number and again ask them to enter their age. In the second while loop, we ask for a password, store the user’s input in password, and break out of the loop if the input was alphanumeric. If it wasn’t, we’re not satisfied so we tell the user the password needs to be alphanumeric and again ask them to enter a password.

When run, the program’s output looks like this:

Enter your age:
forty two
Please enter a number for your age.
Enter your age:
42
Select a new password (letters and numbers only):
secr3t!
Passwords can only have letters and numbers.
Select a new password (letters and numbers only):
secr3t

Calling isdecimal() and isalnum() on variables, we’re able to test whether the values stored in those variables are decimal or not, alphanumeric or not. Here, these tests help us reject the input forty two and accept 42, and reject secr3t! and accept secr3t.

The startswith() and endswith() methods return True if the string value they are called on begins or ends (respectively) with the string passed to the method; otherwise, they return False. Enter the following into the interactive shell:

>>> 'Hello world!'.startswith('Hello')
True
>>> 'Hello world!'.endswith('world!')
True
>>> 'abc123'.startswith('abcdef')
False
>>> 'abc123'.endswith('12')
False
>>> 'Hello world!'.startswith('Hello world!')
True
>>> 'Hello world!'.endswith('Hello world!')
True

These methods are useful alternatives to the == equals operator if you need to check only whether the first or last part of the string, rather than the whole thing, is equal to another string.

The join() method is useful when you have a list of strings that need to be joined together into a single string value. The join() method is called on a string, gets passed a list of strings, and returns a string. The returned string is the concatenation of each string in the passed-in list. For example, enter the following into the interactive shell:

>>> ', '.join(['cats', 'rats', 'bats'])
'cats, rats, bats'
>>> ' '.join(['My', 'name', 'is', 'Simon'])
'My name is Simon'
>>> 'ABC'.join(['My', 'name', 'is', 'Simon'])
'MyABCnameABCisABCSimon'

Notice that the string join() calls on is inserted between each string of the list argument. For example, when join(['cats', 'rats', 'bats']) is called on the ', ' string, the returned string is ‘cats, rats, bats’.

Remember that join() is called on a string value and is passed a list value. (It’s easy to accidentally call it the other way around.) The split() method does the opposite: It’s called on a string value and returns a list of strings. Enter the following into the interactive shell:

>>> 'My name is Simon'.split()
['My', 'name', 'is', 'Simon']

By default, the string 'My name is Simon' is split wherever whitespace characters such as the space, tab, or newline characters are found. These whitespace characters are not included in the strings in the returned list. You can pass a delimiter string to the split() method to specify a different string to split upon. For example, enter the following into the interactive shell:

>>> 'MyABCnameABCisABCSimon'.split('ABC')
['My', 'name', 'is', 'Simon']
>>> 'My name is Simon'.split('m')
['My na', 'e is Si', 'on']

A common use of split() is to split a multiline string along the newline characters. Enter the following into the interactive shell:

>>> spam = '''Dear Alice,
How have you been? I am fine.
There is a container in the fridge
that is labeled "Milk Experiment".

Please do not drink it.
Sincerely,
Bob'''
>>> spam.split('\n')
['Dear Alice,', 'How have you been? I am fine.', 'There is a container in the
fridge', 'that is labeled "Milk Experiment".', '', 'Please do not drink it.',
'Sincerely,', 'Bob']

Passing split() the argument '\n' lets us split the multiline string stored in spam along the newlines and return a list in which each item corresponds to one line of the string.

The rjust() and ljust() string methods return a padded version of the string they are called on, with spaces inserted to justify the text. The first argument to both methods is an integer length for the justified string. Enter the following into the interactive shell:

>>> 'Hello'.rjust(10)
'     Hello'
>>> 'Hello'.rjust(20)
'               Hello'
>>> 'Hello World'.rjust(20)
'         Hello World'
>>> 'Hello'.ljust(10)
'Hello     '

'Hello'.rjust(10) says that we want to right-justify 'Hello' in a string of total length 10. 'Hello' is five characters, so five spaces will be added to its left, giving us a string of 10 characters with 'Hello' justified right.

An optional second argument to rjust() and ljust() will specify a fill character other than a space character. Enter the following into the interactive shell:

>>> 'Hello'.rjust(20, '*')
'***************Hello'
>>> 'Hello'.ljust(20, '-')
'Hello---------------'

The center() string method works like ljust() and rjust() but centers the text rather than justifying it to the left or right. Enter the following into the interactive shell:

>>> 'Hello'.center(20)
'       Hello       '
>>> 'Hello'.center(20, '=')
'=======Hello========'

These methods are especially useful when you need to print tabular data that has the correct spacing. Open a new file editor window and enter the following code, saving it as picnicTable.py:

def printPicnic(itemsDict, leftWidth, rightWidth):
    print('PICNIC ITEMS'.center(leftWidth + rightWidth, '-'))
    for k, v in itemsDict.items():
        print(k.ljust(leftWidth, '.') + str(v).rjust(rightWidth))
picnicItems = {'sandwiches': 4, 'apples': 12, 'cups': 4, 'cookies': 8000}
printPicnic(picnicItems, 12, 5)
printPicnic(picnicItems, 20, 6)

In this program, we define a printPicnic() method that will take in a dictionary of information and use center(), ljust(), and rjust() to display that information in a neatly aligned table-like format.

The dictionary that we’ll pass to printPicnic() is picnicItems. In picnicItems, we have 4 sandwiches, 12 apples, 4 cups, and 8000 cookies. We want to organize this information into two columns, with the name of the item on the left and the quantity on the right.

To do this, we decide how wide we want the left and right columns to be. Along with our dictionary, we’ll pass these values to printPicnic().

printPicnic() takes in a dictionary, a leftWidth for the left column of a table, and a rightWidth for the right column. It prints a title, PICNIC ITEMS, centered above the table. Then, it loops through the dictionary, printing each key-value pair on a line with the key justified left and padded by periods, and the value justified right and padded by spaces.

After defining printPicnic(), we define the dictionary picnicItems and call printPicnic() twice, passing it different widths for the left and right table columns.

When you run this program, the picnic items are displayed twice. The first time the left column is 12 characters wide, and the right column is 5 characters wide. The second time they are 20 and 6 characters wide, respectively.

---PICNIC ITEMS--
sandwiches..    4
apples......   12
cups........    4
cookies..... 8000
-------PICNIC ITEMS-------
sandwiches..........     4
apples..............    12
cups................     4
cookies.............  8000

Using rjust(), ljust(), and center() lets you ensure that strings are neatly aligned, even if you aren’t sure how many characters long your strings are.

Sometimes you may want to strip off whitespace characters (space, tab, and newline) from the left side, right side, or both sides of a string. The strip() string method will return a new string without any whitespace characters at the beginning or end. The lstrip() and rstrip() methods will remove whitespace characters from the left and right ends, respectively. Enter the following into the interactive shell:

>>> spam = '    Hello World     '
>>> spam.strip()
'Hello World'
>>> spam.lstrip()
'Hello World '
>>> spam.rstrip()
'    Hello World'

Optionally, a string argument will specify which characters on the ends should be stripped. Enter the following into the interactive shell:

>>> spam = 'SpamSpamBaconSpamEggsSpamSpam'
>>> spam.strip('ampS')
'BaconSpamEggs'

Passing strip() the argument 'ampS' will tell it to strip occurences of a, m, p, and capital S from the ends of the string stored in spam. The order of the characters in the string passed to strip() does not matter: strip('ampS') will do the same thing as strip('mapS') or strip('Spam').

The pyperclip module has copy() and paste() functions that can send text to and receive text from your computer’s clipboard. Sending the output of your program to the clipboard will make it easy to paste it to an email, word processor, or some other software.

Pyperclip does not come with Python. To install it, follow the directions for installing third-party modules in Appendix A. After installing the pyperclip module, enter the following into the interactive shell:

>>> import pyperclip
>>> pyperclip.copy('Hello world!')
>>> pyperclip.paste()
'Hello world!'

Of course, if something outside of your program changes the clipboard contents, the paste() function will return it. For example, if I copied this sentence to the clipboard and then called paste(), it would look like this:

>>> pyperclip.paste()
'For example, if I copied this sentence to the clipboard and then called
paste(), it would look like this:'

You want to be able to run this program with a command line argument that is the account’s name—for instance, email or blog. That account’s password will be copied to the clipboard so that the user can paste it into a Password field. This way, the user can have long, complicated passwords without having to memorize them.

Open a new file editor window and save the program as pw.py. You need to start the program with a #! (shebang) line (see Appendix B) and should also write a comment that briefly describes the program. Since you want to associate each account’s name with its password, you can store these as strings in a dictionary. The dictionary will be the data structure that organizes your account and password data. Make your program look like the following:

#! python3
# pw.py - An insecure password locker program.

PASSWORDS = {'email': 'F7minlBDDuvMJuxESSKHFhTxFtjVB6',
             'blog': 'VmALvQyKAxiVH5G8v01if1MLZF3sdt',
             'luggage': '12345'}

The command line arguments will be stored in the variable sys.argv. (See Appendix B for more information on how to use command line arguments in your programs.) The first item in the sys.argv list should always be a string containing the program’s filename ('pw.py'), and the second item should be the first command line argument. For this program, this argument is the name of the account whose password you want. Since the command line argument is mandatory, you display a usage message to the user if they forget to add it (that is, if the sys.argv list has fewer than two values in it). Make your program look like the following:

#! python3
# pw.py - An insecure password locker program.

PASSWORDS = {'email': 'F7minlBDDuvMJuxESSKHFhTxFtjVB6',
             'blog': 'VmALvQyKAxiVH5G8v01if1MLZF3sdt',
             'luggage': '12345'}

import sys
if len(sys.argv) < 2:
    print('Usage: python pw.py [account] - copy account password')
    sys.exit()

account = sys.argv[1]      # first command line arg is the account name

Now that the account name is stored as a string in the variable account, you need to see whether it exists in the PASSWORDS dictionary as a key. If so, you want to copy the key’s value to the clipboard using pyperclip.copy(). (Since you’re using the pyperclip module, you need to import it.) Note that you don’t actually need the account variable; you could just use sys.argv[1] everywhere account is used in this program. But a variable named account is much more readable than something cryptic like sys.argv[1].

Make your program look like the following:

#! python3
# pw.py - An insecure password locker program.
PASSWORDS = {'email': 'F7minlBDDuvMJuxESSKHFhTxFtjVB6',
             'blog': 'VmALvQyKAxiVH5G8v01if1MLZF3sdt',
             'luggage': '12345'}

import sys, pyperclip
if len(sys.argv) < 2:
    print('Usage: py pw.py [account] - copy account password')
    sys.exit()

account = sys.argv[1]   # first command line arg is the account name

if account in PASSWORDS:
    pyperclip.copy(PASSWORDS[account])
    print('Password for ' + account + ' copied to clipboard.')
else:
    print('There is no account named ' + account)

This new code looks in the PASSWORDS dictionary for the account name. If the account name is a key in the dictionary, we get the value corresponding to that key, copy it to the clipboard, and print a message saying that we copied the value. Otherwise, we print a message saying there’s no account with that name.

That’s the complete script. Using the instructions in Appendix B for launching command line programs easily, you now have a fast way to copy your account passwords to the clipboard. You will have to modify the PASSWORDS dictionary value in the source whenever you want to update the program with a new password.

Of course, you probably don’t want to keep all your passwords in one place where anyone could easily copy them. But you can modify this program and use it to quickly copy regular text to the clipboard. Say you are sending out several emails that have many of the same stock paragraphs in common. You could put each paragraph as a value in the PASSWORDS dictionary (you’d probably want to rename the dictionary at this point), and then you would have a way to quickly select and copy one of many standard pieces of text to the clipboard.

On Windows, you can create a batch file to run this program with the WIN-R Run window. (For more about batch files, see Appendix B.) Type the following into the file editor and save the file as pw.bat in the C:\Windows folder:

@py.exe C:\Python34\pw.py %*
@pause

With this batch file created, running the password-safe program on Windows is just a matter of pressing WIN-R and typing pw <account name>.

You want the bulletPointAdder.py program to do the following:

That second step is a little tricky, but steps 1 and 3 are pretty straightforward: They just involve the pyperclip.copy() and pyperclip.paste() functions. For now, let’s just write the part of the program that covers steps 1 and 3. Enter the following, saving the program as bulletPointAdder.py:

#! python3
# bulletPointAdder.py - Adds Wikipedia bullet points to the start
# of each line of text on the clipboard.

import pyperclip
text = pyperclip.paste()

# TODO: Separate lines and add stars.

pyperclip.copy(text)

The TODO comment is a reminder that you should complete this part of the program eventually. The next step is to actually implement that piece of the program.

The call to pyperclip.paste() returns all the text on the clipboard as one big string. If we used the “List of Lists of Lists” example, the string stored in text would look like this:

'Lists of animals\nLists of aquarium life\nLists of biologists by author
abbreviation\nLists of cultivars'

The \n newline characters in this string cause it to be displayed with multiple lines when it is printed or pasted from the clipboard. There are many “lines” in this one string value. You want to add a star to the start of each of these lines.

You could write code that searches for each \n newline character in the string and then adds the star just after that. But it would be easier to use the split() method to return a list of strings, one for each line in the original string, and then add the star to the front of each string in the list.

Make your program look like the following:

#! python3
# bulletPointAdder.py - Adds Wikipedia bullet points to the start
# of each line of text on the clipboard.

import pyperclip
text = pyperclip.paste()

# Separate lines and add stars.
lines = text.split('\n')
for i in range(len(lines)):    # loop through all indexes in the "lines" list
    lines[i] = '* ' + lines[i] # add star to each string in "lines" list

pyperclip.copy(text)

We split the text along its newlines to get a list in which each item is one line of the text. We store the list in lines and then loop through the items in lines. For each line, we add a star and a space to the start of the line. Now each string in lines begins with a star.

The lines list now contains modified lines that start with stars. But pyperclip.copy() is expecting a single string value, not a list of string values. To make this single string value, pass lines into the join() method to get a single string joined from the list’s strings. Make your program look like the following:

#! python3
# bulletPointAdder.py - Adds Wikipedia bullet points to the start
# of each line of text on the clipboard.

import pyperclip
text = pyperclip.paste()

# Separate lines and add stars.
lines = text.split('\n')
for i in range(len(lines)):    # loop through all indexes for "lines" list
    lines[i] = '* ' + lines[i] # add star to each string in "lines" list
text = '\n'.join(lines)
pyperclip.copy(text)

When this program is run, it replaces the text on the clipboard with text that has stars at the start of each line. Now the program is complete, and you can try running it with text copied to the clipboard.

Even if you don’t need to automate this specific task, you might want to automate some other kind of text manipulation, such as removing trailing spaces from the end of lines or converting text to uppercase or lowercase. Whatever your needs, you can use the clipboard for input and output.

Write a function named printTable() that takes a list of lists of strings and displays it in a well-organized table with each column right-justified. Assume that all the inner lists will contain the same number of strings. For example, the value could look like this:

tableData = [['apples', 'oranges', 'cherries', 'banana'],
             ['Alice', 'Bob', 'Carol', 'David'],
             ['dogs', 'cats', 'moose', 'goose']]

Your printTable() function would print the following:

  apples Alice  dogs
 oranges   Bob  cats
cherries Carol moose
  banana David goose

Hint: Your code will first have to find the longest string in each of the inner lists so that the whole column can be wide enough to fit all the strings. You can store the maximum width of each column as a list of integers. The printTable() function can begin with colWidths = [0] * len(tableData), which will create a list containing the same number of 0 values as the number of inner lists in tableData. That way, colWidths[0] can store the width of the longest string in tableData[0], colWidths[1] can store the width of the longest string in tableData[1], and so on. You can then find the largest value in the colWidths list to find out what integer width to pass to the rjust() string method.