Download to read offline
![6 CHAPTER 1. WHY SHOULD YOU LEARN TO WRITE PROGRAMS?
print('Hello world!')
And we have even written our first syntactically correct Python sentence. Our
sentence starts with the function print followed by a string of text of our choosing
enclosed in single quotes. The strings in the print statements are enclosed in quotes.
Single quotes and double quotes do the same thing; most people use single quotes
except in cases like this where a single quote (which is also an apostrophe) appears
in the string.
1.5 Conversing with Python
Now that we have a word and a simple sentence that we know in Python, we need
to know how to start a conversation with Python to test our new language skills.
Before you can converse with Python, you must first install the Python software
on your computer and learn how to start Python on your computer. That is too
much detail for this chapter so I suggest that you consult www.py4e.com where
I have detailed instructions and screencasts of setting up and starting Python on
Macintosh and Windows systems. At some point, you will be in a terminal or
command window and you will type python and the Python interpreter will start
executing in interactive mode and appear somewhat as follows:
Python 3.5.1 (v3.5.1:37a07cee5969, Dec 6 2015, 01:54:25)
[MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more
information.
>>>
The >>> prompt is the Python interpreter’s way of asking you, “What do you want
me to do next?” Python is ready to have a conversation with you. All you have
to know is how to speak the Python language.
Let’s say for example that you did not know even the simplest Python language
words or sentences. You might want to use the standard line that astronauts use
when they land on a faraway planet and try to speak with the inhabitants of the
planet:
>>> I come in peace, please take me to your leader
File "<stdin>", line 1
I come in peace, please take me to your leader
^
SyntaxError: invalid syntax
>>>
This is not going so well. Unless you think of something quickly, the inhabitants
of the planet are likely to stab you with their spears, put you on a spit, roast you
over a fire, and eat you for dinner.
Luckily you brought a copy of this book on your travels, and you thumb to this
very page and try again:](https://image.slidesharecdn.com/pythonforeverybody-210824144749/85/Python-for-everybody-18-320.jpg)
![6 CHAPTER 1. WHY SHOULD YOU LEARN TO WRITE PROGRAMS?
print('Hello world!')
And we have even written our first syntactically correct Python sentence. Our
sentence starts with the function print followed by a string of text of our choosing
enclosed in single quotes. The strings in the print statements are enclosed in quotes.
Single quotes and double quotes do the same thing; most people use single quotes
except in cases like this where a single quote (which is also an apostrophe) appears
in the string.
1.5 Conversing with Python
Now that we have a word and a simple sentence that we know in Python, we need
to know how to start a conversation with Python to test our new language skills.
Before you can converse with Python, you must first install the Python software
on your computer and learn how to start Python on your computer. That is too
much detail for this chapter so I suggest that you consult www.py4e.com where
I have detailed instructions and screencasts of setting up and starting Python on
Macintosh and Windows systems. At some point, you will be in a terminal or
command window and you will type python and the Python interpreter will start
executing in interactive mode and appear somewhat as follows:
Python 3.5.1 (v3.5.1:37a07cee5969, Dec 6 2015, 01:54:25)
[MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more
information.
>>>
The >>> prompt is the Python interpreter’s way of asking you, “What do you want
me to do next?” Python is ready to have a conversation with you. All you have
to know is how to speak the Python language.
Let’s say for example that you did not know even the simplest Python language
words or sentences. You might want to use the standard line that astronauts use
when they land on a faraway planet and try to speak with the inhabitants of the
planet:
>>> I come in peace, please take me to your leader
File "<stdin>", line 1
I come in peace, please take me to your leader
^
SyntaxError: invalid syntax
>>>
This is not going so well. Unless you think of something quickly, the inhabitants
of the planet are likely to stab you with their spears, put you on a spit, roast you
over a fire, and eat you for dinner.
Luckily you brought a copy of this book on your travels, and you thumb to this
very page and try again:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-18-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![1.6. TERMINOLOGY: INTERPRETER AND COMPILER 9
>>> print(y)
42
>>>
In this example, we ask Python to remember the value six and use the label x so
we can retrieve the value later. We verify that Python has actually remembered
the value using print. Then we ask Python to retrieve x and multiply it by seven
and put the newly computed value in y. Then we ask Python to print out the
value currently in y.
Even though we are typing these commands into Python one line at a time, Python
is treating them as an ordered sequence of statements with later statements able
to retrieve data created in earlier statements. We are writing our first simple
paragraph with four sentences in a logical and meaningful order.
It is the nature of an interpreter to be able to have an interactive conversation
as shown above. A compiler needs to be handed the entire program in a file,
and then it runs a process to translate the high-level source code into machine
language and then the compiler puts the resulting machine language into a file for
later execution.
If you have a Windows system, often these executable machine language programs
have a suffix of “.exe” or “.dll” which stand for “executable” and “dynamic link
library” respectively. In Linux and Macintosh, there is no suffix that uniquely
marks a file as executable.
If you were to open an executable file in a text editor, it would look completely
crazy and be unreadable:
^?ELF^A^A^A^@^@^@^@^@^@^@^@^@^B^@^C^@^A^@^@^@xa0x82
^D^H4^@^@^@x90^]^@^@^@^@^@^@4^@ ^@^G^@(^@$^@!^@^F^@
^@^@4^@^@^@4x80^D^H4x80^D^Hxe0^@^@^@xe0^@^@^@^E
^@^@^@^D^@^@^@^C^@^@^@^T^A^@^@^Tx81^D^H^Tx81^D^H^S
^@^@^@^S^@^@^@^D^@^@^@^A^@^@^@^A^D^HQVhTx83^D^Hxe8
....
It is not easy to read or write machine language, so it is nice that we have inter-
preters and compilers that allow us to write in high-level languages like Python or
C.
Now at this point in our discussion of compilers and interpreters, you should be
wondering a bit about the Python interpreter itself. What language is it written
in? Is it written in a compiled language? When we type “python”, what exactly
is happening?
The Python interpreter is written in a high-level language called “C”. You can look
at the actual source code for the Python interpreter by going to www.python.org
and working your way to their source code. So Python is a program itself and it
is compiled into machine code. When you installed Python on your computer (or
the vendor installed it), you copied a machine-code copy of the translated Python
program onto your system. In Windows, the executable machine code for Python
itself is likely in a file with a name like:
C:Python35python.exe](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-21-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![1.9. THE BUILDING BLOCKS OF PROGRAMS 11
the clown ran after the car and the car ran into the tent
and the tent fell down on the clown and the car
Then imagine that you are doing this task looking at millions of lines of text.
Frankly it would be quicker for you to learn Python and write a Python program
to count the words than it would be to manually scan the words.
The even better news is that I already came up with a simple program to find the
most common word in a text file. I wrote it, tested it, and now I am giving it to
you to use so you can save some time.
name = input('Enter file:')
handle = open(name, 'r')
counts = dict()
for line in handle:
words = line.split()
for word in words:
counts[word] = counts.get(word, 0) + 1
bigcount = None
bigword = None
for word, count in list(counts.items()):
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
print(bigword, bigcount)
# Code: http://www.py4e.com/code3/words.py
You don’t even need to know Python to use this program. You will need to
get through Chapter 10 of this book to fully understand the awesome Python
techniques that were used to make the program. You are the end user, you simply
use the program and marvel at its cleverness and how it saved you so much manual
effort. You simply type the code into a file called words.py and run it or you
download the source code from http://www.py4e.com/code3/ and run it.
This is a good example of how Python and the Python language are acting as an
intermediary between you (the end user) and me (the programmer). Python is a
way for us to exchange useful instruction sequences (i.e., programs) in a common
language that can be used by anyone who installs Python on their computer. So
neither of us are talking to Python, instead we are communicating with each other
through Python.
1.9 The building blocks of programs
In the next few chapters, we will learn more about the vocabulary, sentence struc-
ture, paragraph structure, and story structure of Python. We will learn about the](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-23-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![4.6. ADDING NEW FUNCTIONS 47
To choose an element from a sequence at random, you can use choice:
t = [1, 2, 3]
random.choice(t)
2
random.choice(t)
3
The random module also provides functions to generate random values from con-
tinuous distributions including Gaussian, exponential, gamma, and a few more.
4.6 Adding new functions
So far, we have only been using the functions that come with Python, but it is also
possible to add new functions. A function definition specifies the name of a new
function and the sequence of statements that execute when the function is called.
Once we define a function, we can reuse the function over and over throughout our
program.
Here is an example:
def print_lyrics():
print(I'm a lumberjack, and I'm okay.)
print('I sleep all night and I work all day.')
def is a keyword that indicates that this is a function definition. The name of
the function is print_lyrics. The rules for function names are the same as for
variable names: letters, numbers and some punctuation marks are legal, but the
first character can’t be a number. You can’t use a keyword as the name of a
function, and you should avoid having a variable and a function with the same
name.
The empty parentheses after the name indicate that this function doesn’t take any
arguments. Later we will build functions that take arguments as their inputs.
The first line of the function definition is called the header; the rest is called
the body. The header has to end with a colon and the body has to be indented.
By convention, the indentation is always four spaces. The body can contain any
number of statements.
If you type a function definition in interactive mode, the interpreter prints ellipses
(. . . ) to let you know that the definition isn’t complete:
def print_lyrics():
... print(I'm a lumberjack, and I'm okay.)
... print('I sleep all night and I work all day.')
...
To end the function, you have to enter an empty line (this is not necessary in a
script).
Defining a function creates a variable with the same name.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-59-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![60 CHAPTER 5. ITERATION
while True:
line = input(' ')
if line[0] == '#':
continue
if line == 'done':
break
print(line)
print('Done!')
# Code: http://www.py4e.com/code3/copytildone2.py
Here is a sample run of this new program with continue added.
hello there
hello there
# don't print this
print this!
print this!
done
Done!
All the lines are printed except the one that starts with the hash sign because
when the continue is executed, it ends the current iteration and jumps back to
the while statement to start the next iteration, thus skipping the print statement.
5.5 Definite loops using for
Sometimes we want to loop through a set of things such as a list of words, the lines
in a file, or a list of numbers. When we have a list of things to loop through, we
can construct a definite loop using a for statement. We call the while statement
an indefinite loop because it simply loops until some condition becomes False,
whereas the for loop is looping through a known set of items so it runs through
as many iterations as there are items in the set.
The syntax of a for loop is similar to the while loop in that there is a for
statement and a loop body:
friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends:
print('Happy New Year:', friend)
print('Done!')
In Python terms, the variable friends is a list1
of three strings and the for loop
goes through the list and executes the body once for each of the three strings in
the list resulting in this output:
1We will examine lists in more detail in a later chapter.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-72-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![5.6. LOOP PATTERNS 61
Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Done!
Translating this for loop to English is not as direct as the while, but if you think
of friends as a set, it goes like this: “Run the statements in the body of the for
loop once for each friend in the set named friends.”
Looking at the for loop, for and in are reserved Python keywords, and friend
and friends are variables.
for friend in friends:
print('Happy New Year:', friend)
In particular, friend is the iteration variable for the for loop. The variable friend
changes for each iteration of the loop and controls when the for loop completes.
The iteration variable steps successively through the three strings stored in the
friends variable.
5.6 Loop patterns
Often we use a for or while loop to go through a list of items or the contents of
a file and we are looking for something such as the largest or smallest value of the
data we scan through.
These loops are generally constructed by:
• Initializing one or more variables before the loop starts
• Performing some computation on each item in the loop body, possibly chang-
ing the variables in the body of the loop
• Looking at the resulting variables when the loop completes
We will use a list of numbers to demonstrate the concepts and construction of these
loop patterns.
5.6.1 Counting and summing loops
For example, to count the number of items in a list, we would write the following
for loop:
count = 0
for itervar in [3, 41, 12, 9, 74, 15]:
count = count + 1
print('Count: ', count)](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-73-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![62 CHAPTER 5. ITERATION
We set the variable count to zero before the loop starts, then we write a for loop
to run through the list of numbers. Our iteration variable is named itervar and
while we do not use itervar in the loop, it does control the loop and cause the
loop body to be executed once for each of the values in the list.
In the body of the loop, we add 1 to the current value of count for each of the
values in the list. While the loop is executing, the value of count is the number
of values we have seen “so far”.
Once the loop completes, the value of count is the total number of items. The
total number “falls in our lap” at the end of the loop. We construct the loop so
that we have what we want when the loop finishes.
Another similar loop that computes the total of a set of numbers is as follows:
total = 0
for itervar in [3, 41, 12, 9, 74, 15]:
total = total + itervar
print('Total: ', total)
In this loop we do use the iteration variable. Instead of simply adding one to the
count as in the previous loop, we add the actual number (3, 41, 12, etc.) to the
running total during each loop iteration. If you think about the variable total, it
contains the “running total of the values so far”. So before the loop starts total is
zero because we have not yet seen any values, during the loop total is the running
total, and at the end of the loop total is the overall total of all the values in the
list.
As the loop executes, total accumulates the sum of the elements; a variable used
this way is sometimes called an accumulator.
Neither the counting loop nor the summing loop are particularly useful in practice
because there are built-in functions len() and sum() that compute the number of
items in a list and the total of the items in the list respectively.
5.6.2 Maximum and minimum loops
To find the largest value in a list or sequence, we construct the following loop:
largest = None
print('Before:', largest)
for itervar in [3, 41, 12, 9, 74, 15]:
if largest is None or itervar largest :
largest = itervar
print('Loop:', itervar, largest)
print('Largest:', largest)
When the program executes, the output is as follows:
Before: None
Loop: 3 3](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-74-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![5.6. LOOP PATTERNS 63
Loop: 41 41
Loop: 12 41
Loop: 9 41
Loop: 74 74
Loop: 15 74
Largest: 74
The variable largest is best thought of as the “largest value we have seen so far”.
Before the loop, we set largest to the constant None. None is a special constant
value which we can store in a variable to mark the variable as “empty”.
Before the loop starts, the largest value we have seen so far is None since we have
not yet seen any values. While the loop is executing, if largest is None then we
take the first value we see as the largest so far. You can see in the first iteration
when the value of itervar is 3, since largest is None, we immediately set largest
to be 3.
After the first iteration, largest is no longer None, so the second part of the
compound logical expression that checks itervar largest triggers only when
we see a value that is larger than the “largest so far”. When we see a new “even
larger” value we take that new value for largest. You can see in the program
output that largest progresses from 3 to 41 to 74.
At the end of the loop, we have scanned all of the values and the variable largest
now does contain the largest value in the list.
To compute the smallest number, the code is very similar with one small change:
smallest = None
print('Before:', smallest)
for itervar in [3, 41, 12, 9, 74, 15]:
if smallest is None or itervar smallest:
smallest = itervar
print('Loop:', itervar, smallest)
print('Smallest:', smallest)
Again, smallest is the “smallest so far” before, during, and after the loop executes.
When the loop has completed, smallest contains the minimum value in the list.
Again as in counting and summing, the built-in functions max() and min() make
writing these exact loops unnecessary.
The following is a simple version of the Python built-in min() function:
def min(values):
smallest = None
for value in values:
if smallest is None or value smallest:
smallest = value
return smallest
In the function version of the smallest code, we removed all of the print statements
so as to be equivalent to the min function which is already built in to Python.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-75-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![Chapter 6
Strings
6.1 A string is a sequence
A string is a sequence of characters. You can access the characters one at a time
with the bracket operator:
fruit = 'banana'
letter = fruit[1]
The second statement extracts the character at index position 1 from the fruit
variable and assigns it to the letter variable.
The expression in brackets is called an index. The index indicates which character
in the sequence you want (hence the name).
But you might not get what you expect:
print(letter)
a
For most people, the first letter of “banana” is “b”, not “a”. But in Python, the
index is an offset from the beginning of the string, and the offset of the first letter
is zero.
letter = fruit[0]
print(letter)
b
So “b” is the 0th letter (“zero-th”) of “banana”, “a” is the 1th letter (“one-th”),
and “n” is the 2th (“two-th”) letter.
You can use any expression, including variables and operators, as an index, but
the value of the index has to be an integer. Otherwise you get:
letter = fruit[1.5]
TypeError: string indices must be integers
67](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-79-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![68 CHAPTER 6. STRINGS
b
[0]
a
[1]
n
[2]
a
[3]
n
[4]
a
[5]
Figure 6.1: String Indexes
6.2 Getting the length of a string using len
len is a built-in function that returns the number of characters in a string:
fruit = 'banana'
len(fruit)
6
To get the last letter of a string, you might be tempted to try something like this:
length = len(fruit)
last = fruit[length]
IndexError: string index out of range
The reason for the IndexError is that there is no letter in “banana” with the index
6. Since we started counting at zero, the six letters are numbered 0 to 5. To get
the last character, you have to subtract 1 from length:
last = fruit[length-1]
print(last)
a
Alternatively, you can use negative indices, which count backward from the end of
the string. The expression fruit[-1] yields the last letter, fruit[-2] yields the
second to last, and so on.
6.3 Traversal through a string with a loop
A lot of computations involve processing a string one character at a time. Often
they start at the beginning, select each character in turn, do something to it, and
continue until the end. This pattern of processing is called a traversal. One way
to write a traversal is with a while loop:
index = 0
while index len(fruit):
letter = fruit[index]
print(letter)
index = index + 1](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-80-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![6.4. STRING SLICES 69
This loop traverses the string and displays each letter on a line by itself. The
loop condition is index len(fruit), so when index is equal to the length of
the string, the condition is false, and the body of the loop is not executed. The
last character accessed is the one with the index len(fruit)-1, which is the last
character in the string.
Exercise 1: Write a while loop that starts at the last character in the
string and works its way backwards to the first character in the string,
printing each letter on a separate line, except backwards.
Another way to write a traversal is with a for loop:
for char in fruit:
print(char)
Each time through the loop, the next character in the string is assigned to the
variable char. The loop continues until no characters are left.
6.4 String slices
A segment of a string is called a slice. Selecting a slice is similar to selecting a
character:
s = 'Monty Python'
print(s[0:5])
Monty
print(s[6:12])
Python
The operator returns the part of the string from the “n-th” character to the “m-th”
character, including the first but excluding the last.
If you omit the first index (before the colon), the slice starts at the beginning of
the string. If you omit the second index, the slice goes to the end of the string:
fruit = 'banana'
fruit[:3]
'ban'
fruit[3:]
'ana'
If the first index is greater than or equal to the second the result is an empty string,
represented by two quotation marks:
fruit = 'banana'
fruit[3:3]
''
An empty string contains no characters and has length 0, but other than that, it
is the same as any other string.
Exercise 2: Given that fruit is a string, what does fruit[:] mean?](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-81-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![70 CHAPTER 6. STRINGS
6.5 Strings are immutable
It is tempting to use the operator on the left side of an assignment, with the
intention of changing a character in a string. For example:
greeting = 'Hello, world!'
greeting[0] = 'J'
TypeError: 'str' object does not support item assignment
The “object” in this case is the string and the “item” is the character you tried
to assign. For now, an object is the same thing as a value, but we will refine that
definition later. An item is one of the values in a sequence.
The reason for the error is that strings are immutable, which means you can’t
change an existing string. The best you can do is create a new string that is a
variation on the original:
greeting = 'Hello, world!'
new_greeting = 'J' + greeting[1:]
print(new_greeting)
Jello, world!
This example concatenates a new first letter onto a slice of greeting. It has no
effect on the original string.
6.6 Looping and counting
The following program counts the number of times the letter “a” appears in a
string:
word = 'banana'
count = 0
for letter in word:
if letter == 'a':
count = count + 1
print(count)
This program demonstrates another pattern of computation called a counter. The
variable count is initialized to 0 and then incremented each time an “a” is found.
When the loop exits, count contains the result: the total number of a’s.
Exercise 3: Encapsulate this code in a function named count, and gen-
eralize it so that it accepts the string and the letter as arguments.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-82-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![72 CHAPTER 6. STRINGS
stuff = 'Hello world'
type(stuff)
class 'str'
dir(stuff)
['capitalize', 'casefold', 'center', 'count', 'encode',
'endswith' , 'expandtabs', 'find', 'format', 'format_map',
'index' , 'isalnum', 'isalpha', 'isdecimal', 'isdigit',
'isidentifier' , 'islower', 'isnumeric', 'isprintable',
'isspace' , 'istitle', 'isupper', 'join', 'ljust', 'lower',
'lstrip' , 'maketrans', 'partition', 'replace', 'rfind',
'rindex' , 'rjust', 'rpartition', 'rsplit', 'rstrip',
'split' , 'splitlines', 'startswith', 'strip', 'swapcase',
'title' , 'translate', 'upper', 'zfill']
help(str.capitalize)
Help on method_descriptor:
capitalize(...)
S.capitalize() - str
Return a capitalized version of S, i.e. make the first character
have upper case and the rest lower case.
While the dir function lists the methods, and you can use help to get some simple
documentation on a method, a better source of documentation for string methods
would be https://docs.python.org/library/stdtypes.html#string-methods.
Calling a method is similar to calling a function (it takes arguments and returns
a value) but the syntax is different. We call a method by appending the method
name to the variable name using the period as a delimiter.
For example, the method upper takes a string and returns a new string with all
uppercase letters:
Instead of the function syntax upper(word), it uses the method syntax
word.upper().
word = 'banana'
new_word = word.upper()
print(new_word)
BANANA
This form of dot notation specifies the name of the method, upper, and the name
of the string to apply the method to, word. The empty parentheses indicate that
this method takes no argument.
A method call is called an invocation; in this case, we would say that we are
invoking upper on the word.
For example, there is a string method named find that searches for the position
of one string within another:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-84-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![74 CHAPTER 6. STRINGS
Exercise 4: There is a string method called count that is similar to
the function in the previous exercise. Read the documentation of this
method at:
https://docs.python.org/library/stdtypes.html#string-methods
Write an invocation that counts the number of times the letter a occurs
in “banana”.
6.10 Parsing strings
Often, we want to look into a string and find a substring. For example if we were
presented a series of lines formatted as follows:
From stephen.marquard@ uct.ac.za Sat Jan 5 09:14:16 2008
and we wanted to pull out only the second half of the address (i.e., uct.ac.za)
from each line, we can do this by using the find method and string slicing.
First, we will find the position of the at-sign in the string. Then we will find the
position of the first space after the at-sign. And then we will use string slicing to
extract the portion of the string which we are looking for.
data = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008'
atpos = data.find('@')
print(atpos)
21
sppos = data.find(' ',atpos)
print(sppos)
31
host = data[atpos+1:sppos]
print(host)
uct.ac.za
We use a version of the find method which allows us to specify a position in
the string where we want find to start looking. When we slice, we extract the
characters from “one beyond the at-sign through up to but not including the space
character”.
The documentation for the find method is available at
https://docs.python.org/library/stdtypes.html#string-methods.
6.11 Format operator
The format operator, % allows us to construct strings, replacing parts of the strings
with the data stored in variables. When applied to integers, % is the modulus
operator. But when the first operand is a string, % is the format operator.
The first operand is the format string, which contains one or more format sequences
that specify how the second operand is formatted. The result is a string.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-86-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![76 CHAPTER 6. STRINGS
while True:
line = input(' ')
if line[0] == '#':
continue
if line == 'done':
break
print(line)
print('Done!')
# Code: http://www.py4e.com/code3/copytildone2.py
Look what happens when the user enters an empty line of input:
hello there
hello there
# don't print this
print this!
print this!
Traceback (most recent call last):
File copytildone.py, line 3, in module
if line[0] == '#':
IndexError: string index out of range
The code works fine until it is presented an empty line. Then there is no zero-th
character, so we get a traceback. There are two solutions to this to make line three
“safe” even if the line is empty.
One possibility is to simply use the startswith method which returns False if
the string is empty.
if line.startswith('#'):
Another way is to safely write the if statement using the guardian pattern and
make sure the second logical expression is evaluated only where there is at least
one character in the string.:
if len(line) 0 and line[0] == '#':
6.13 Glossary
counter A variable used to count something, usually initialized to zero and then
incremented.
empty string A string with no characters and length 0, represented by two quo-
tation marks.
format operator An operator, %, that takes a format string and a tuple and gen-
erates a string that includes the elements of the tuple formatted as specified
by the format string.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-88-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![6.14. EXERCISES 77
format sequence A sequence of characters in a format string, like %d, that spec-
ifies how a value should be formatted.
format string A string, used with the format operator, that contains format
sequences.
flag A boolean variable used to indicate whether a condition is true or false.
invocation A statement that calls a method.
immutable The property of a sequence whose items cannot be assigned.
index An integer value used to select an item in a sequence, such as a character
in a string.
item One of the values in a sequence.
method A function that is associated with an object and called using dot notation.
object Something a variable can refer to. For now, you can use “object” and
“value” interchangeably.
search A pattern of traversal that stops when it finds what it is looking for.
sequence An ordered set; that is, a set of values where each value is identified by
an integer index.
slice A part of a string specified by a range of indices.
traverse To iterate through the items in a sequence, performing a similar opera-
tion on each.
6.14 Exercises
Exercise 5: Take the following Python code that stores a string:
str = 'X-DSPAM-Confidence:0.8475'
Use find and string slicing to extract the portion of the string after the
colon character and then use the float function to convert the extracted
string into a floating point number.
Exercise 6: Read the documentation of the string methods at
https://docs.python.org/library/stdtypes.html#string-methods You
might want to experiment with some of them to make sure you
understand how they work. strip and replace are particularly useful.
The documentation uses a syntax that might be confusing. For example,
in find(sub[, start[, end]]), the brackets indicate optional arguments.
So sub is required, but start is optional, and if you include start, then
end is optional.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-89-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![80 CHAPTER 7. FILES
We will primarily focus on reading and writing text files such as those we create in
a text editor. Later we will see how to work with database files which are binary
files, specifically designed to be read and written through database software.
7.2 Opening files
When we want to read or write a file (say on your hard drive), we first must
open the file. Opening the file communicates with your operating system, which
knows where the data for each file is stored. When you open a file, you are
asking the operating system to find the file by name and make sure the file exists.
In this example, we open the file mbox.txt, which should be stored in the same
folder that you are in when you start Python. You can download this file from
www.py4e.com/code3/mbox.txt
fhand = open('mbox.txt')
print(fhand)
_io.TextIOWrapper name='mbox.txt' mode='r' encoding='cp1252'
If the open is successful, the operating system returns us a file handle. The file
handle is not the actual data contained in the file, but instead it is a “handle” that
we can use to read the data. You are given a handle if the requested file exists and
you have the proper permissions to read the file.
From stephen.m..
Return-Path: p..
Date: Sat, 5 Jan ..
To: source@coll..
From: stephen...
Subject: [sakai]...
Details: http:/...
…
Your
Program
H
A
N
D
L
E
open
close
read
write
Figure 7.2: A File Handle
If the file does not exist, open will fail with a traceback and you will not get a
handle to access the contents of the file:
fhand = open('stuff.txt')
Traceback (most recent call last):
File stdin, line 1, in module
FileNotFoundError: [Errno 2] No such file or directory: 'stuff.txt'
Later we will use try and except to deal more gracefully with the situation where
we attempt to open a file that does not exist.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-92-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![7.3. TEXT FILES AND LINES 81
7.3 Text files and lines
A text file can be thought of as a sequence of lines, much like a Python string can
be thought of as a sequence of characters. For example, this is a sample of a text
file which records mail activity from various individuals in an open source project
development team:
From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008
Return-Path: postmaster@collab.sakaiproject.org
Date: Sat, 5 Jan 2008 09:12:18 -0500
To: source@collab.sakaiproject.org
From: stephen.marquard@uct.ac.za
Subject: [sakai] svn commit: r39772 - content/branches/
Details: http://source.sakaiproject.org/viewsvn/?view=revrev=39772
...
The entire file of mail interactions is available from
www.py4e.com/code3/mbox.txt
and a shortened version of the file is available from
www.py4e.com/code3/mbox-short.txt
These files are in a standard format for a file containing multiple mail messages.
The lines which start with “From” separate the messages and the lines which start
with “From:” are part of the messages. For more information about the mbox
format, see https://en.wikipedia.org/wiki/Mbox.
To break the file into lines, there is a special character that represents the “end of
the line” called the newline character.
In Python, we represent the newline character as a backslash-n in string constants.
Even though this looks like two characters, it is actually a single character. When
we look at the variable by entering “stuff” in the interpreter, it shows us the n
in the string, but when we use print to show the string, we see the string broken
into two lines by the newline character.
stuff = 'HellonWorld!'
stuff
'HellonWorld!'
print(stuff)
Hello
World!
stuff = 'XnY'
print(stuff)
X
Y
len(stuff)
3
You can also see that the length of the string XnY is three characters because the
newline character is a single character.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-93-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![82 CHAPTER 7. FILES
So when we look at the lines in a file, we need to imagine that there is a special
invisible character called the newline at the end of each line that marks the end of
the line.
So the newline character separates the characters in the file into lines.
7.4 Reading files
While the file handle does not contain the data for the file, it is quite easy to
construct a for loop to read through and count each of the lines in a file:
fhand = open('mbox-short.txt')
count = 0
for line in fhand:
count = count + 1
print('Line Count:', count)
# Code: http://www.py4e.com/code3/open.py
We can use the file handle as the sequence in our for loop. Our for loop simply
counts the number of lines in the file and prints them out. The rough translation
of the for loop into English is, “for each line in the file represented by the file
handle, add one to the count variable.”
The reason that the open function does not read the entire file is that the file might
be quite large with many gigabytes of data. The open statement takes the same
amount of time regardless of the size of the file. The for loop actually causes the
data to be read from the file.
When the file is read using a for loop in this manner, Python takes care of splitting
the data in the file into separate lines using the newline character. Python reads
each line through the newline and includes the newline as the last character in the
line variable for each iteration of the for loop.
Because the for loop reads the data one line at a time, it can efficiently read and
count the lines in very large files without running out of main memory to store
the data. The above program can count the lines in any size file using very little
memory since each line is read, counted, and then discarded.
If you know the file is relatively small compared to the size of your main memory,
you can read the whole file into one string using the read method on the file handle.
fhand = open('mbox-short.txt')
inp = fhand.read()
print(len(inp))
94626
print(inp[:20])
From stephen.marquar
In this example, the entire contents (all 94,626 characters) of the file mbox-short.txt
are read directly into the variable inp. We use string slicing to print out the first
20 characters of the string data stored in inp.](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-94-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
![86 CHAPTER 7. FILES
We read the file name from the user and place it in a variable named fname and
open that file. Now we can run the program repeatedly on different files.
python search6.py
Enter the file name: mbox.txt
There were 1797 subject lines in mbox.txt
python search6.py
Enter the file name: mbox-short.txt
There were 27 subject lines in mbox-short.txt
Before peeking at the next section, take a look at the above program and ask
yourself, “What could go possibly wrong here?” or “What might our friendly user
do that would cause our nice little program to ungracefully exit with a traceback,
making us look not-so-cool in the eyes of our users?”
7.7 Using try, except, and open
I told you not to peek. This is your last chance.
What if our user types something that is not a file name?
python search6.py
Enter the file name: missing.txt
Traceback (most recent call last):
File search6.py, line 2, in module
fhand = open(fname)
FileNotFoundError: [Errno 2] No such file or directory: 'missing.txt'
python search6.py
Enter the file name: na na boo boo
Traceback (most recent call last):
File search6.py, line 2, in module
fhand = open(fname)
FileNotFoundError: [Errno 2] No such file or directory: 'na na boo boo'
Do not laugh. Users will eventually do every possible thing they can do to break
your programs, either on purpose or with malicious intent. As a matter of fact,
an important part of any software development team is a person or group called
Quality Assurance (or QA for short) whose very job it is to do the craziest things
possible in an attempt to break the software that the programmer has created.
The QA team is responsible for finding the flaws in programs before we have
delivered the program to the end users who may be purchasing the software or
paying our salary to write the software. So the QA team is the programmer’s best
friend.
So now that we see the flaw in the program, we can elegantly fix it using the
try/except structure. We need to assume that the open call might fail and add
recovery code when the open fails as follows:](/p?url=https%3A%2F%2Fimage.slidesharecdn.com%2Fpythonforeverybody-210824144749%2F85%2FPython-for-everybody-98-320.jpg&__src=https%3A%2F%2Fwww.slideshare.net%2Fslideshow%2Fpython-for-everybody-250040858%2F250040858&__type=image)
Uploaded byNageswararao Kuchipudi
Python for everybody
This document is the preface to the book "Python for Everybody" by Dr. Charles R. Severance. It discusses how the book was created by remixing and adapting the existing book "Think Python" under a more open license, with the goal of shifting the focus from computer science to data analysis using Python. Major changes were made to reorder and replace topics with a data-oriented approach. The preface provides background on the motivation and process for creating this new textbook.
![[PPT] _ Unit 2 _ 9.0 _ Domain Specific IoT _Home Automation.pdf](https://cdn.slidesharecdn.com/ss_thumbnails/pptunit29-220516115946-098632b6-thumbnail.jpg?width=640&height=640&fit=bounds)
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