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Manipulating Strings
137
strings in a dictionary. The dictionary will be the data structure that orga-
nizes 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'}
Step 2: Handle Command Line Arguments
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
Step 3: Copy the Right Password
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]
every-
where
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.
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Chapter 6
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 correspond-
ing 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 pro-
gram 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
dic-
tionary (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>
.
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Manipulating Strings
139
Project: Adding Bullets to wiki markup
When editing a Wikipedia article, you can create a bulleted list by putting
each list item on its own line and placing a star in front. But say you have
a really large list that you want to add bullet points to. You could just type
those stars at the beginning of each line, one by one. Or you could auto-
mate this task with a short Python script.
The bulletPointAdder.py script will get the text from the clipboard, add
a star and space to the beginning of each line, and then paste this new
text to the clipboard. For example, if I copied the following text (for the
Wikipedia article “List of Lists of Lists”) to the clipboard:
Lists of animals
Lists of aquarium life
Lists of biologists by author abbreviation
Lists of cultivars
and then ran the bulletPointAdder.py program, the clipboard would then con-
tain the following:
* Lists of animals
* Lists of aquarium life
* Lists of biologists by author abbreviation
* Lists of cultivars
This star-prefixed text is ready to be pasted into a Wikipedia article as a
bulleted list.
Step 1: Copy and Paste from the Clipboard
You want the bulletPointAdder.py program to do the following:
1. Paste text from the clipboard
2. Do something to it
3. Copy the new text to the clipboard
That second step is a little tricky, but steps 1 and 3 are pretty straight-
forward: They just involve the
pyperclip.copy()
and
pyperclip.paste()
func-
tions. 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()
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Chapter 6
# 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.
Step 2: Separate the Lines of Text and Add the Star
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.
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Manipulating Strings
141
Step 3: Join the Modified Lines
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.
Summary
Text is a common form of data, and Python comes with many helpful string
methods to process the text stored in string values. You will make use of
indexing, slicing, and string methods in almost every Python program you
write.
The programs you are writing now don’t seem too sophisticated—they
don’t have graphical user interfaces with images and colorful text. So far,
you’re displaying text with
print()
and letting the user enter text with
input()
.
However, the user can quickly enter large amounts of text through the clip-
board. This ability provides a useful avenue for writing programs that manip-
ulate massive amounts of text. These text-based programs might not have
flashy windows or graphics, but they can get a lot of useful work done quickly.
Another way to manipulate large amounts of text is reading and writing
files directly off the hard drive. You’ll learn how to do this with Python in
the next chapter.
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142
Chapter 6
Practice Questions
1. What are escape characters?
2. What do the
\n
and
\t
escape characters represent?
3. How can you put a
\
backslash character in a string?
4. The string value
"Howl's Moving Castle"
is a valid string. Why isn’t it
a problem that the single quote character in the word
Howl's
isn’t
escaped?
5. If you don’t want to put
\n
in your string, how can you write a string
with newlines in it?
6. What do the following expressions evaluate to?
•
'Hello world!'[1]
•
'Hello world!'[0:5]
•
'Hello world!'[:5]
•
'Hello world!'[3:]
7. What do the following expressions evaluate to?
•
'Hello'.upper()
•
'Hello'.upper().isupper()
•
'Hello'.upper().lower()
8. What do the following expressions evaluate to?
•
'Remember, remember, the fifth of November.'.split()
•
'-'.join('There can be only one.'.split())
9. What string methods can you use to right-justify, left-justify, and center
a string?
10. How can you trim whitespace characters from the beginning or end of
a string?
Practice Project
For practice, write a program that does the following.
Table Printer
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']]
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Manipulating Strings
143
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 lon-
gest 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.
2
Part II
Au t o m A t i n g t A S k S
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