Part 2
The essentials
I
n the chapters that follow, we’ll show you the essentials of Python. We’ll start
from the absolute basics of what makes a Python program and move through
Python’s built-in data types and control structures, as well as defining functions
and using modules.
The last section of this part moves on to show you how to write standalone
Python programs, manipulate files, handle errors, and use classes. The section
ends with chapter 16, which is a brief introduction to GUI programming using
Python’s tkinter module.
The absolute basics
This chapter covers
■
Indenting and block structuring
■
Differentiating comments
■
Assigning variables
■
Evaluating expressions
■
Using common data types
■
Getting user input
■
Using correct Pythonic style
This chapter describes the absolute basics in Python: assignments and expressions,
how to type a number or a string, how to indicate comments in code, and so forth.
It starts out with a discussion of how Python block structures its code, which is different from any other major language.
4.1
Indentation and block structuring
Python differs from most other programming languages because it uses whitespace
and indentation to determine block structure (that is, to determine what constitutes
the body of a loop, the else clause of a conditional, and so on). Most languages use
35
36
CHAPTER 4
The absolute basics
braces of some sort to do this. Here is C code that calculates the factorial of 9, leaving
the result in the variable r:
/* This is C code
int n, r;
n = 9;
r = 1;
while (n > 0) {
r *= n;
n--;
}
*/
The { and } delimit the body of the while loop, the code that is executed with each
repetition of the loop. The code is usually indented more or less as shown, to make
clear what’s going on, but it could also be written like this:
/* And this is C code with arbitrary indentation */
int n, r;
n = 9;
r = 1;
while (n > 0) {
r *= n;
n--;
}
It still would execute correctly, even though it’s rather difficult to read.
Here’s the Python equivalent:
# This is
n = 9
r = 1
while n >
r = r
n = n
Python code. (Yea!)
0:
* n
- 1
Python also supports
C-style r *= n
Python also supports
n -= 1
Python doesn’t use braces to indicate code structure; instead, the indentation itself is
used. The last two lines of the previous code are the body of the while loop because
they come immediately after the while statement and are indented one level further
than the while statement. If they weren’t indented, they wouldn’t be part of the body
of the while.
Using indentation to structure code rather than braces may take some getting used
to, but there are significant benefits:
It’s impossible to have missing or extra braces. You’ll never need to hunt
through your code for the brace near the bottom that matches the one a few
lines from the top.
The visual structure of the code reflects its real structure. This makes it easy to
grasp the skeleton of code just by looking at it.
Python coding styles are mostly uniform. In other words, you’re unlikely to go
crazy from dealing with someone’s idea of aesthetically pleasing code. Their
code will look pretty much like yours.
Variables and assignments
37
You probably use consistent indentation in your code already, so this won’t be a big
step for you. If you’re using IDLE, it automatically indents lines. You just need to backspace out of levels of indentation when desired. Most programming editors and IDEs,
including Emacs, VIM, and Eclipse, to name a few, provide this functionality as well.
One thing that may trip you up once or twice until you get used to it is that the Python
interpreter returns an error message if you have a space (or spaces) preceding the
commands you enter at a prompt.
4.2
Differentiating comments
For the most part, anything following a # symbol in a Python file is a comment and is
disregarded by the language. The obvious exception is a # in a string, which is just a
character of that string:
#
x
x
x
Assign 5 to x
= 5
= 3
# Now x is 3
= "# This is not a comment"
We’ll put comments into Python code frequently.
4.3
Variables and assignments
The most commonly used command in Python is assignment, which looks pretty close
to what you might’ve used in other languages. Python code to create a variable called
x and assign the value 5 to that variable is
x = 5
In Python, neither a variable type declaration nor an end-of-line delimiter is necessary,
unlike in many other computer languages. The line is ended by the end of the line.
Variables are created automatically when they’re first assigned.
Python variables can be set to any object, unlike C or many other languages’ variables, which can store only the type of value they’re declared as. The following is perfectly legal Python code:
>>> x = "Hello"
>>> print(x)
Hello
>>> x = 5
>>> print(x)
5
x starts out referring to the string object "Hello" and then refers to the integer object
5. Of course, this feature can be abused because arbitrarily assigning the same vari-
able name to refer successively to different data types can make code confusing to
understand.
A new assignment overrides any previous assignments. The del statement deletes
the variable. Trying to print the variable’s contents after deleting it gives an error the
same as if the variable had never been created in the first place.
38
CHAPTER 4
The absolute basics
>>> x = 5
>>> print(x)
5
>>> del x
>>> print(x)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>>
Here we have our first look at a traceback, which is printed when an error, called an
exception, has been detected. The last line tells us what exception was detected, which
in this case is a NameError exception on x. After its deletion, x is no longer a valid variable name. In this example, the trace returns only line 1, in <module> because only
the single line has been sent in the interactive mode. In general, the full dynamic call
structure of the existing function calls at the time of the occurrence of the error is
returned. If you’re using IDLE, you obtain the same information with some small differences; it may look something like this:
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
print(x)
NameError: name 'x' is not defined
Chapter 14 will describe this mechanism in more detail. A full list of the possible
exceptions and what causes them is in the appendix of this book. Use the index to
find any specific exception (such as NameError) you receive.
Variable names are case sensitive and can include any alphanumeric character as
well as underscores but must start with a letter or underscore. See section 4.10 for
more guidance on the Pythonic style for creating variable names.
4.4
Expressions
Python supports arithmetic and similar expressions; these will be familiar to most
readers. The following code calculates the average of 3 and 5, leaving the result in the
variable z:
x = 3
y = 5
z = (x + y) / 2
Note that unlike the arithmetic rules of C in terms of type coercions, arithmetic operators involving only integers do not always return an integer. Even though all the values are integers, division (starting with Python 3) returns a floating-point number, so
the fractional part isn’t truncated. If you want traditional integer division returning a
truncated integer, you can use the // instead.
Standard rules of arithmetic precedence apply; if we’d left out the parentheses in
the last line, it would’ve been calculated as x + (y / 2).
Strings
39
Expressions don’t have to involve just numerical values; strings, Boolean values,
and many other types of objects can be used in expressions in various ways. We’ll discuss these in more detail as they’re used.
4.5
Strings
You’ve already seen that Python, like most other programming languages, indicates
strings through the use of double quotes. This line leaves the string "Hello, World"
in the variable x:
x = "Hello, World"
Backslashes can be used to escape characters, to give them special meanings. \n
means the newline character, \t means the tab character, \\ means a single normal
backslash character, and \" is a plain double-quote character. It doesn’t end the
string:
x = "\tThis string starts with a \"tab\"."
x = "This string contains a single backslash(\\)."
You can use single quotes instead of double quotes. The following two lines do the
same thing:
x = "Hello, World"
x = 'Hello, World'
The only difference is that you don’t need to backslash " characters in single-quoted
strings or ' characters in double-quoted strings:
x
x
x
x
=
=
=
=
"Don't need a backslash"
'Can\'t get by without a backslash'
"Backslash your \" character!"
'You can leave the " alone'
You can’t split a normal string across lines; this code won’t work:
# This Python code will cause an ERROR -- you can't split the string
across two lines.
x = "This is a misguided attempt to
put a newline into a string without using backslash-n"
But Python offers triple-quoted strings, which let you do this and permit single and
double quotes to be included without backslashes:
x = """Starting and ending a string with triple " characters
permits embedded newlines, and the use of " and ' without
backslashes"""
Now x is the entire sentence between the """ delimiters. (You can also use triple single quotes—'''—instead of triple double quotes to do the same thing.)
Python offers enough string-related functionality that chapter 6 is devoted to the
topic.
40
4.6
CHAPTER 4
The absolute basics
Numbers
Because you’re probably familiar with standard numeric operations from other languages, this book doesn’t contain a separate chapter describing Python’s numeric
abilities. This section describes the unique features of Python numbers, and the
Python documentation lists the available functions.
Python offers four kinds of numbers: integers, floats, complex numbers, and Booleans.
An integer constant is written as an integer—0, –11, +33, 123456—and has unlimited
range, restricted only by the resources of your machine. A float can be written with a
decimal point or using scientific notation: 3.14, –2E-8, 2.718281828. The precision of
these values is governed by the underlying machine but is typically equal to double
(64-bit) types in C. Complex numbers are probably of limited interest and are discussed separately later in the section. Booleans are either True or False and behave
identically to 1 and 0 except for their string representations.
Arithmetic is much like it is in C. Operations involving two integers produce an
integer, except for division (/), where a float results. If the // division symbol is used,
the result is an integer, with truncation. Operations involving a float always produce a
float. Here are a few examples:
>>> 5 + 2 - 3 * 2
1
>>> 5 / 2
# floating point result with normal division
2.5
>>> 5 / 2.0
# also a floating point result
2.5
>>> 5 // 2
# integer result with truncation when divided using '//'
2
>>> 30000000000
# This would be too large to be an int in many languages
30000000000
>>> 30000000000 * 3
90000000000
>>> 30000000000 * 3.0
90000000000.0
>>> 2.0e-8
# Scientific notation gives back a float
2e-08
>>> 3000000 * 3000000
9000000000000
>>> int(200.2)
200
>>> int(2e2)
200
>>> float(200)
200.0
q
q
q
These are explicit conversions between types q. int will truncate float values.
Numbers in Python have two advantages over C or Java. First, integers can be arbitrarily large; and second, the division of two integers results in a float.
Numbers
4.6.1
41
Built-in numeric functions
Python provides the following number-related functions as part of its core:
abs, divmod, cmp, coerce, float, hex, int, long, max, min, oct,
pow, round
See the documentation for details.
4.6.2
Advanced numeric functions
More advanced numeric functions such as the trig and hyperbolic trig functions, as
well as a few useful constants, aren’t built-ins in Python but are provided in a standard
module called math. Modules will be explained in detail later; for now, it’s sufficient to
know that the math functions in this section must be made available by starting your
Python program or interactive session with the statement
from math import *
The math module provides the following functions and constants:
acos, asin, atan, atan2, ceil, cos, cosh, e, exp, fabs, floor, fmod,
frexp, hypot, ldexp, log, log10, mod, pi, pow, sin, sinh, sqrt, tan,
tanh
See the documentation for details.
4.6.3
Numeric computation
The core Python installation isn’t well suited to intensive numeric computation
because of speed constraints. But the powerful Python extension NumPy provides
highly efficient implementations of many advanced numeric operations. The emphasis is on array operations, including multidimensional matrices and more advanced
functions such as the Fast Fourier Transform. You should be able to find NumPy (or
links to it) at www.scipy.org.
4.6.4
Complex numbers
Complex numbers are created automatically whenever an expression of the form nj is
encountered, with n having the same form as a Python integer or float. j is, of course,
standard engineering notation for the imaginary number equal to the square root of
–1, for example:
>>> (3+2j)
(3+2j)
Note that Python expresses the resulting complex number in parentheses, as a way of
indicating that what is printed to the screen represents the value of a single object:
>>> 3 + 2j - (4+4j)
(-1-2j)
42
CHAPTER 4
The absolute basics
>>> (1+2j) * (3+4j)
(-5+10j)
>>> 1j * 1j
(-1+0j)
Calculating j * j gives the expected answer of –1, but the result remains a Python
complex number object. Complex numbers are never converted automatically to
equivalent real or integer objects. But you can easily access their real and imaginary
parts with real and imag:
>>> z = (3+5j)
>>> z.real
3.0
>>> z.imag
5.0
Note that real and imaginary parts of a complex number are always returned as floatingpoint numbers.
4.6.5
Advanced complex-number functions
The functions in the math module don’t apply to complex numbers; the rationale is
that most users want the square root of –1 to generate an error, not an answer!
Instead, similar functions, which can operate on complex numbers, are provided in
the cmath module:
acos, acosh, asin, asinh, atan, atanh, cos, cosh, e, exp, log, log10,
pi, sin, sinh, sqrt, tan, tanh.
In order to make clear in the code that these are special-purpose complex-number
functions and to avoid name conflicts with the more normal equivalents, it’s best to
import the cmath module by saying
import cmath
and then to explicitly refer to the cmath package when using the function:
>>> import cmath
>>> cmath.sqrt(-1)
1j
Minimizing from <module> import *
This is a good example of why it’s best to minimize the use of the from <module>
import * form of the import statement. If you imported first the math and then the
cmath modules using it, the commonly named functions in cmath would override those
of math. It’s also more work for someone reading your code to figure out the source
of the specific functions you use. Some modules are explicitly designed to use this
form of import.
See chapter 10 for more details on how to use modules and module names.
Basic Python style
43
The important thing to keep in mind is that by importing the cmath module, you can
do almost anything you can do with other numbers.
4.7
The None value
In addition to standard types such as strings and numbers, Python has a special basic
data type that defines a single special data object called None. As the name suggests,
None is used to represent an empty value. It appears in various guises throughout
Python. For example, a procedure in Python is just a function that doesn’t explicitly
return a value, which means that, by default, it returns None.
None is often useful in day-to-day Python programming as a placeholder, to indicate a point in a data structure where meaningful data will eventually be found, even
though that data hasn’t yet been calculated. You can easily test for the presence of
None, because there is only one instance of None in the entire Python system (all references to None point to the same object), and None is equivalent only to itself.
4.8
Getting input from the user
You can also use the input() function to get input from the user. Use the prompt
string you want displayed to the user as input’s parameter:
>>> name = input("Name? ")
Name? Vern
>>> print(name)
Vern
>>> age = int(input("Age? "))
Age? 28
>>> print(age)
28
>>>
Converts input
from string to int
This is a fairly simple way to get user input. The one catch is that the input comes in as
a string, so if you want to use it as a number, you have to use the int() or float()
function to convert it.
4.9
Built-in operators
Python provides various built-in operators, from the standard (such as +, *, and so on)
to the more esoteric, such as operators for performing bit shifting, bitwise logical
functions, and so forth. Most of these operators are no more unique to Python than to
any other language, and hence I won’t explain them in the main text. You can find a
complete list of the Python built-in operators in the documentation.
4.10 Basic Python style
Python has relatively few limitations on coding style with the obvious exception of the
requirement to use indentation to organize code into blocks. Even in that case, the
amount of indentation and type of indentation (tabs versus spaces) isn’t mandated.
However, there are preferred stylistic conventions for Python, which are contained in
44
CHAPTER 4
The absolute basics
Python Enhancement Proposal (PEP) 8, which is summarized in the appendix and
can be found online at www.python.org/dev/peps/pep-0008/. A selection of Pythonic
conventions is provided in table 4.1, but to fully absorb Pythonic style you’ll need to
periodically reread PEP 8.
Table 4.1
Pythonic coding conventions
Situation
Suggestion
Example
Module/package names
short, all lowercase, underscores only if
needed
imp, sys
Function names
all lowercase, underscores_for_readablitiy
foo(), my_func()
Variable names
all lowercase, underscores_for_readablitiy
my_var
Class names
CapitalizeEachWord
MyClass
Constant names
ALL_CAPS_WITH_UNDERSCORES
PI, TAX_RATE
Indentation
4 spaces per level, don’t use tabs
Comparisons
Don't compare explicitly to True or False
if my_var:
if not my_var:
I strongly urge you to follow the conventions of PEP 8. They’re wisely chosen and time
tested and will make your code easier for you and other Python programmers to
understand.
4.11 Summary
That’s the view of Python from 30,000 feet. If you’re an experienced programmer,
you’re probably already seeing how you can write your code in Python. If that’s the case,
you should feel free to start experimenting with your own code. Many programmers
find it surprisingly easy to pick up Python syntax, because there are relatively few surprises. Once you pick up the basics of the language, it’s very predictable and consistent.
In any case, we have just covered the broadest outlines of the language, and there
are lots of details that we still need to cover, beginning in the next chapter with one of
the workhorses of Python, lists.
PROGRAMMING
THE Quick Python Book SECOND EDITION
Vernon L. Ceder
T
his revision of Manning’s popular The Quick Python Book
offers a clear, crisp introduction to the elegant Python
programming language and its famously easy-to-read syntax.
Written for programmers new to Python, this updated edition
covers features common to other languages concisely, while
introducing Python’s comprehensive standard functions library
and unique features in detail.
After exploring Python’s syntax, control flow, and basic data
structures, the book shows how to create, test, and deploy full
applications and larger code libraries. It addresses established
Python features as well as the advanced object-oriented options
available in Python 3. Along the way, you’ll survey the current
Python development landscape, including GUI programming,
testing, database access, and web frameworks.
What’s Inside
Concepts and Python 3 features
Regular expressions and testing
Python tools
All the Python you need— nothing you don’t
Second edition author Vern Ceder is Director of Technology at
the Canterbury School in Fort Wayne, Indiana where he teaches
and uses Python. The first edition of this book was written by
Daryl Harms and Kenneth McDonald.
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