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The name of the func-**
tion is a variable that refers to a function object.
**header The first line of a function definition.**
**import statement A statement that reads a module file and creates a module**
object.
**module object A value created by an import statement that provides access to**
the data and code de... |
If a function call is used as an expression,**
the return value is the value of the expression.
**void function A function that does not return a value.**
##### 4.14 Exercises
Exercise 4: What is the purpose of the “def” keyword in Python?
a) It is slang that means “the following code is really cool”
b) It indicate... |
Repeating identical or
similar tasks without making errors is something that computers do well and people
do poorly. |
Because iteration is so common, Python provides several language
features to make it easier.
One form of iteration in Python is the while statement. |
Here is a simple program
that counts down from five and then says “Blastoff!”.
-----
n = 5
**while n > 0:**
print(n)
n = n - 1
print('Blastoff!')
You can almost read the while statement as if it were English. |
It means, “While
n is greater than 0, display the value of n and then reduce the value of n by 1.
When you get to 0, exit the while statement and display the word Blastoff!”
More formally, here is the flow of execution for a while statement:
1. |
Evaluate the condition, yielding True or False.
2. If the condition is false, exit the while statement and continue execution at
the next statement.
3. |
If the condition is true, execute the body and then go back to step 1.
This type of flow is called a loop because the third step loops back around to the
top. |
We call each time we execute the body of the loop an iteration. |
For the above
loop, we would say, “It had five iterations”, which means that the body of the loop
was executed five times.
The body of the loop should change the value of one or more variables so that
eventually the condition becomes false and the loop terminates. |
We call the variable that changes each time the loop executes and controls when the loop finishes
the iteration variable. |
If there is no iteration variable, the loop will repeat forever,
resulting in an infinite loop.
##### 5.3 Infinite loops
An endless source of amusement for programmers is the observation that the directions on shampoo, “Lather, rinse, repeat,” are an infinite loop because there is
no iteration variable telling you ho... |
Other times a loop is
obviously infinite because it has no iteration variable at all.
##### 5.4 “Infinite loops” and break
Sometimes you don’t know it’s time to end a loop until you get half way through
the body. |
In that case you can write an infinite loop on purpose and then use the
break statement to jump out of the loop.
This loop is obviously an infinite loop because the logical expression on the while
statement is simply the logical constant True:
-----
n = 10
**while True:**
print(n, end=' ')
n = n - 1
print('Done!'... |
This program will run forever or until your battery runs out
because the logical expression at the top of the loop is always true by virtue of the
fact that the expression is the constant value True.
While this is a dysfunctional infinite loop, we can still use this pattern to build
useful loops as long as we carefull... |
If the user types
done, the break statement exits the loop. Otherwise the program echoes whatever
the user types and goes back to the top of the loop. |
Here’s a sample run:
- hello there
hello there
- finished
finished
- done
Done!
This way of writing while loops is common because you can check the condition
anywhere in the loop (not just at the top) and you can express the stop condition
affirmatively (“stop when this happens”) rather than negatively (“keep going... |
In that case you can use the continue
-----
statement to skip to the next iteration without finishing the body of the loop for
the current iteration.
Here is an example of a loop that copies its input until the user types “done”, but
treats lines that start with the hash character as lines not to be printed (kind o... |
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
state... |
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.
##### 5.7 Loop patterns
Often we use a for or while loop to go through a list of items or the contents of
a file and ... |
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... |
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 t... |
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('Befor... |
More code means more chances to make an error and more places for
bugs to hide.
One way to cut your debugging time is “debugging by bisection.” For example, if
there are 100 lines in your program and you check them one at a time, it would
take 100 steps.
Instead, try to break the problem in half. |
Look at the middle of the program,
or near it, for an intermediate value you can check. |
Add a print statement (or
something else that has a verifiable effect) and run the program.
If the mid-point check is incorrect, the problem must be in the first half of the
program. |
If it is correct, the problem is in the second half.
Every time you perform a check like this, you halve the number of lines you have
to search. |
After six steps (which is much less than 100), you would be down to
one or two lines of code, at least in theory.
In practice it is not always clear what the “middle of the program” is and not
always possible to check it. |
It doesn’t make sense to count lines and find the exact
midpoint. Instead, think about places in the program where there might be errors
and places where it is easy to put a check. |
Then choose a spot where you think
the chances are about the same that the bug is before or after the check.
##### 5.9 Glossary
**accumulator A variable used in a loop to add up or accumulate a result.**
**counter A variable used in a loop to count the number of times something hap-**
pened. |
We initialize a counter to zero and then increment the counter each
time we want to “count” something.
**decrement An update that decreases the value of a variable.**
**initialize An assignment that gives an initial value to a variable that will be**
updated.
**increment An update that increases the value of a varia... |
Once “done” is entered, print out the total, count, and average of the
numbers. |
If the user enters anything other than a number, detect their mistake
using try and except and print an error message and skip to the next number.
Enter a number: 4
Enter a number: 5
Enter a number: bad data
Invalid input
Enter a number: 7
Enter a number: done
16 3 5.333333333333333
Exercise 2: Write another program ... |
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-eth”) of “banana”, a is the 1th letter (“one-eth”), and
n is the 2th (“two-eth”) letter.
You can use any expression, including va... |
Otherwise you get:
>>> letter = fruit[1.5]
TypeError: string indices must be integers
-----
## 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 ret... |
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
-----
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.
Ano... |
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-eth” character to the
“m-eth” character, including the first but excluding the last.
If you omit the first index (before the colon), ... |
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 strin... |
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... |
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 generalize it so that it
accepts the string and the letter as arguments.
-----
##### 6.7 The... |
To see if two strings are equal:
**if word == 'banana':**
print('All right, bananas.')
Other comparison operations are useful for putting words in alphabetical order:
**if word < 'banana':**
print('Your word,' + word + ', comes before banana.')
**elif word > 'banana':**
print('Your word,' + word + ', comes after... |
All the uppercase letters come before all the lowercase letters, so:
Your word, Pineapple, comes before banana.
A common way to address this problem is to convert strings to a standard format,
such as all lowercase, before performing the comparison. |
Keep that in mind in case
you have to defend yourself against a man armed with a Pineapple.
##### 6.9 string methods
Strings are an example of Python objects. |
An object contains both data (the actual
string itself) and methods, which are effectively functions that are built into the
object and are available to any instance of the object.
Python has a function called dir which lists the methods available for an object.
The type function shows the type of an object and the di... |
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/3.5/library/stdtypes.html#string-methods.](https://doc... |
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 = ... |
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:
-----
>>> word = 'banana'
>>> in... |
As long as we are
careful with the order, we can make multiple method calls in a single expression.
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/3.5/library/stdtypes.html#string-methods and write an](https://docs.python.org/3.5/library/stdtypes.html#string-methods)
invocation that counts the number of times the letter a occurs in “banana”.
-----
##### 6.10 Parsing strings
Often, we want to l... |
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 positio... |
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... |
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/3.5/library/stdtypes.html#string-methods.](https://docs.python.org/3.5/library/stdtypes.html#string-methods)
#####... |
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.
For example, the format sequence “%d” means that the second operand should be
formatted as an integer (d stands for “decimal”):
>>> camels = 42
>>> '%d' % camels
_'42'_
-----
The result is the string “42”, which is not to be confused with the integer value
42.
A format sequence can appear ... |
Each format sequence is matched with an element of the tuple, in
order.
The following example uses “%d” to format an integer, “%g” to format a floatingpoint number (don’t ask why), and “%s” to format a string:
>>> 'In %d years I have spotted %g %s.' % (3, 0.1, 'camels')
_'In 3 years I have spotted 0.1 camels.'_
The ... |
The types of the elements also must match the format sequences:
>>> '%d %d %d' % (1, 2)
TypeError: not enough arguments for format string
>>> '%d' % 'dollars'
TypeError: %d format: a number is required, not str
In the first example, there aren’t enough elements; in the second, the element is
the wrong type.
The form... |
You can read more
about it at
[https://docs.python.org/3.5/library/stdtypes.html#printf-style-string-formatting.](https://docs.python.org/3.5/library/stdtypes.html#printf-style-string-formatting)
##### 6.12 Debugging
A skill that you should cultivate as you program is always asking yourself, “What
could go wrong her... |
We will cover
tuples in Chapter 10
-----
**if line[0] == '#':**
**continue**
**if line == 'done':**
**break**
print(line)
print('Done!')
_# Code: http://www.pythonlearn.com/code3/copytildone2.py_
Look what happens when the user enters an empty line of input:
- hello there
hello there
- # don't print this
- ... |
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 ... |
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.**
**tr... |
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.
-----
## Chapter 7
# Files
##### 7.1 Persistence
So far, we have learned how to write programs and communicate our intentions to
the Central Processing Unit using conditional execution, functions, and iterations.
We have le... |
The
CPU and memory are where our software works and runs. |
It is where all of the
“thinking” happens.
But if you recall from our hardware architecture discussions, once the power is
turned off, anything stored in either the CPU or main memory is erased. |
So up to
now, our programs have just been transient fun exercises to learn Python.
###### Secondary Memory
Figure 7.1: Secondary Memory
In this chapter, we start to work with Secondary Memory (or files). |
Secondary
memory is not erased when the power is turned off. |
Or in the case of a USB flash
drive, the data we write from our programs can be removed from the system and
transported to another system.
-----
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.pythonlearn.com/code3/mbox.txt](http://www.pythonlearn.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.
|open|H A N D L E|Col3|
|---|---|---|
|close|||
|read|||
|write|||
|open H A close N D read L write E Your Program|From stephen.m.. |
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