3. An Informal Introduction to Python¶
In the following examples, input and output are distinguished by the presence or absence of prompts (>>> and ...): to repeat the example, you must type everything after the prompt, when the prompt appears; lines that do not begin with a prompt are output from the interpreter. Note that a secondary prompt on a line by itself in an example means you must type a blank line; this is used to end a multi-line command.
Many of the examples in this manual, even those entered at the interactive
prompt, include comments. Comments in Python start with the hash character,
#, and extend to the end of the physical line. A comment may appear at the
start of a line or following whitespace or code, but not within a string
literal. A hash character within a string literal is just a hash character.
Since comments are to clarify code and are not interpreted by Python, they may
be omitted when typing in examples.
# this is the first comment spam = 1 # and this is the second comment # ... and now a third! text = "# This is not a comment because it's inside quotes."
3.1. Using Python as a Calculator¶
Let’s try some simple Python commands. Start the interpreter and wait for the
>>>. (It shouldn’t take long.)
The interpreter acts as a simple calculator: you can type an expression at it
and it will write the value. Expression syntax is straightforward: the
/ work just like in most other languages
(for example, Pascal or C); parentheses (
()) can be used for grouping.
>>> 2 + 2 4 >>> 50 - 5*6 20 >>> (50 - 5.0*6) / 4 5.0 >>> 8 / 5.0 1.6
The return type of a division (
/) operation depends on its operands. If
both operands are of type
int, floor division is performed
int is returned. If either operand is a
classic division is performed and a
float is returned. The
operator is also provided for doing floor division no matter what the
operands are. The remainder can be calculated with the
>>> 17 / 3 # int / int -> int 5 >>> 17 / 3.0 # int / float -> float 5.666666666666667 >>> 17 // 3.0 # explicit floor division discards the fractional part 5.0 >>> 17 % 3 # the % operator returns the remainder of the division 2 >>> 5 * 3 + 2 # result * divisor + remainder 17
With Python, it is possible to use the
** operator to calculate powers :
>>> 5 ** 2 # 5 squared 25 >>> 2 ** 7 # 2 to the power of 7 128
The equal sign (
=) is used to assign a value to a variable. Afterwards, no
result is displayed before the next interactive prompt:
>>> width = 20 >>> height = 5 * 9 >>> width * height 900
If a variable is not “defined” (assigned a value), trying to use it will give you an error:
>>> n # try to access an undefined variable Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'n' is not defined
There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:
>>> 3 * 3.75 / 1.5 7.5 >>> 7.0 / 2 3.5
In interactive mode, the last printed expression is assigned to the variable
_. This means that when you are using Python as a desk calculator, it is
somewhat easier to continue calculations, for example:
>>> tax = 12.5 / 100 >>> price = 100.50 >>> price * tax 12.5625 >>> price + _ 113.0625 >>> round(_, 2) 113.06
This variable should be treated as read-only by the user. Don’t explicitly assign a value to it — you would create an independent local variable with the same name masking the built-in variable with its magic behavior.
In addition to
float, Python supports other types of
numbers, such as
Python also has built-in support for complex numbers,
and uses the
J suffix to indicate the imaginary part
Besides numbers, Python can also manipulate strings, which can be expressed
in several ways. They can be enclosed in single quotes (
double quotes (
"...") with the same result .
\ can be used
to escape quotes:
>>> 'spam eggs' # single quotes 'spam eggs' >>> 'doesn\'t' # use \' to escape the single quote... "doesn't" >>> "doesn't" # ...or use double quotes instead "doesn't" >>> '"Yes," he said.' '"Yes," he said.' >>> "\"Yes,\" he said." '"Yes," he said.' >>> '"Isn\'t," she said.' '"Isn\'t," she said.'
In the interactive interpreter, the output string is enclosed in quotes and
special characters are escaped with backslashes. While this might sometimes
look different from the input (the enclosing quotes could change), the two
strings are equivalent. The string is enclosed in double quotes if
the string contains a single quote and no double quotes, otherwise it is
enclosed in single quotes. The
>>> '"Isn\'t," she said.' '"Isn\'t," she said.' >>> print '"Isn\'t," she said.' "Isn't," she said. >>> s = 'First line.\nSecond line.' # \n means newline >>> s # without print, \n is included in the output 'First line.\nSecond line.' >>> print s # with print, \n produces a new line First line. Second line.
If you don’t want characters prefaced by
\ to be interpreted as
special characters, you can use raw strings by adding an
the first quote:
>>> print 'C:\some\name' # here \n means newline! C:\some ame >>> print r'C:\some\name' # note the r before the quote C:\some\name
String literals can span multiple lines. One way is using triple-quotes:
'''...'''. End of lines are automatically
included in the string, but it’s possible to prevent this by adding a
the end of the line. The following example:
print """\ Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to """
produces the following output (note that the initial newline is not included):
Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to
Strings can be concatenated (glued together) with the
+ operator, and
>>> # 3 times 'un', followed by 'ium' >>> 3 * 'un' + 'ium' 'unununium'
Two or more string literals (i.e. the ones enclosed between quotes) next to each other are automatically concatenated.
>>> 'Py' 'thon' 'Python'
This only works with two literals though, not with variables or expressions:
>>> prefix = 'Py' >>> prefix 'thon' # can't concatenate a variable and a string literal ... SyntaxError: invalid syntax >>> ('un' * 3) 'ium' ... SyntaxError: invalid syntax
If you want to concatenate variables or a variable and a literal, use
>>> prefix + 'thon' 'Python'
This feature is particularly useful when you want to break long strings:
>>> text = ('Put several strings within parentheses ' 'to have them joined together.') >>> text 'Put several strings within parentheses to have them joined together.'
Strings can be indexed (subscripted), with the first character having index 0. There is no separate character type; a character is simply a string of size one:
>>> word = 'Python' >>> word # character in position 0 'P' >>> word # character in position 5 'n'
Indices may also be negative numbers, to start counting from the right:
>>> word[-1] # last character 'n' >>> word[-2] # second-last character 'o' >>> word[-6] 'P'
Note that since -0 is the same as 0, negative indices start from -1.
In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters, slicing allows you to obtain a substring:
>>> word[0:2] # characters from position 0 (included) to 2 (excluded) 'Py' >>> word[2:5] # characters from position 2 (included) to 5 (excluded) 'tho'
Note how the start is always included, and the end always excluded. This
makes sure that
s[:i] + s[i:] is always equal to
>>> word[:2] + word[2:] 'Python' >>> word[:4] + word[4:] 'Python'
Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.
>>> word[:2] # character from the beginning to position 2 (excluded) 'Py' >>> word[4:] # characters from position 4 (included) to the end 'on' >>> word[-2:] # characters from the second-last (included) to the end 'on'
One way to remember how slices work is to think of the indices as pointing between characters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string of n characters has index n, for example:
+---+---+---+---+---+---+ | P | y | t | h | o | n | +---+---+---+---+---+---+ 0 1 2 3 4 5 6 -6 -5 -4 -3 -2 -1
The first row of numbers gives the position of the indices 0...6 in the string; the second row gives the corresponding negative indices. The slice from i to j consists of all characters between the edges labeled i and j, respectively.
For non-negative indices, the length of a slice is the difference of the
indices, if both are within bounds. For example, the length of
Attempting to use an index that is too large will result in an error:
>>> word # the word only has 6 characters Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: string index out of range
However, out of range slice indexes are handled gracefully when used for slicing:
>>> word[4:42] 'on' >>> word[42:] ''
Python strings cannot be changed — they are immutable. Therefore, assigning to an indexed position in the string results in an error:
>>> word = 'J' ... TypeError: 'str' object does not support item assignment >>> word[2:] = 'py' ... TypeError: 'str' object does not support item assignment
If you need a different string, you should create a new one:
>>> 'J' + word[1:] 'Jython' >>> word[:2] + 'py' 'Pypy'
The built-in function
len() returns the length of a string:
>>> s = 'supercalifragilisticexpialidocious' >>> len(s) 34
- Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange
- Strings, and the Unicode strings described in the next section, are examples of sequence types, and support the common operations supported by such types.
- String Methods
- Both strings and Unicode strings support a large number of methods for basic transformations and searching.
- Format String Syntax
- Information about string formatting with
- String Formatting Operations
- The old formatting operations invoked when strings and Unicode strings are
the left operand of the
%operator are described in more detail here.
3.1.3. Unicode Strings¶
Starting with Python 2.0 a new data type for storing text data is available to the programmer: the Unicode object. It can be used to store and manipulate Unicode data (see http://www.unicode.org/) and integrates well with the existing string objects, providing auto-conversions where necessary.
Unicode has the advantage of providing one ordinal for every character in every
script used in modern and ancient texts. Previously, there were only 256
possible ordinals for script characters. Texts were typically bound to a code
page which mapped the ordinals to script characters. This lead to very much
confusion especially with respect to internationalization (usually written as
'i' + 18 characters +
'n') of software. Unicode solves
these problems by defining one code page for all scripts.
Creating Unicode strings in Python is just as simple as creating normal strings:
>>> u'Hello World !' u'Hello World !'
'u' in front of the quote indicates that a Unicode string is
supposed to be created. If you want to include special characters in the string,
you can do so by using the Python Unicode-Escape encoding. The following
example shows how:
>>> u'Hello\u0020World !' u'Hello World !'
The escape sequence
\u0020 indicates to insert the Unicode character with
the ordinal value 0x0020 (the space character) at the given position.
Other characters are interpreted by using their respective ordinal values directly as Unicode ordinals. If you have literal strings in the standard Latin-1 encoding that is used in many Western countries, you will find it convenient that the lower 256 characters of Unicode are the same as the 256 characters of Latin-1.
For experts, there is also a raw mode just like the one for normal strings. You
have to prefix the opening quote with ‘ur’ to have Python use the
Raw-Unicode-Escape encoding. It will only apply the above
conversion if there is an uneven number of backslashes in front of the small
>>> ur'Hello\u0020World !' u'Hello World !' >>> ur'Hello\\u0020World !' u'Hello\\\\u0020World !'
The raw mode is most useful when you have to enter lots of backslashes, as can be necessary in regular expressions.
Apart from these standard encodings, Python provides a whole set of other ways of creating Unicode strings on the basis of a known encoding.
The built-in function
unicode() provides access to all registered Unicode
codecs (COders and DECoders). Some of the more well known encodings which these
codecs can convert are Latin-1, ASCII, UTF-8, and UTF-16. The latter two
are variable-length encodings that store each Unicode character in one or more
bytes. The default encoding is normally set to ASCII, which passes through
characters in the range 0 to 127 and rejects any other characters with an error.
When a Unicode string is printed, written to a file, or converted with
str(), conversion takes place using this default encoding.
>>> u"abc" u'abc' >>> str(u"abc") 'abc' >>> u"äöü" u'\xe4\xf6\xfc' >>> str(u"äöü") Traceback (most recent call last): File "<stdin>", line 1, in ? UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
To convert a Unicode string into an 8-bit string using a specific encoding,
Unicode objects provide an
encode() method that takes one argument, the
name of the encoding. Lowercase names for encodings are preferred.
>>> u"äöü".encode('utf-8') '\xc3\xa4\xc3\xb6\xc3\xbc'
If you have data in a specific encoding and want to produce a corresponding
Unicode string from it, you can use the
unicode() function with the
encoding name as the second argument.
>>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc', 'utf-8') u'\xe4\xf6\xfc'
Python knows a number of compound data types, used to group together other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.
>>> squares = [1, 4, 9, 16, 25] >>> squares [1, 4, 9, 16, 25]
Like strings (and all other built-in sequence type), lists can be indexed and sliced:
>>> squares # indexing returns the item 1 >>> squares[-1] 25 >>> squares[-3:] # slicing returns a new list [9, 16, 25]
All slice operations return a new list containing the requested elements. This means that the following slice returns a new (shallow) copy of the list:
>>> squares[:] [1, 4, 9, 16, 25]
Lists also supports operations like concatenation:
>>> squares + [36, 49, 64, 81, 100] [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>> cubes = [1, 8, 27, 65, 125] # something's wrong here >>> 4 ** 3 # the cube of 4 is 64, not 65! 64 >>> cubes = 64 # replace the wrong value >>> cubes [1, 8, 27, 64, 125]
You can also add new items at the end of the list, by using
append() method (we will see more about methods later):
>>> cubes.append(216) # add the cube of 6 >>> cubes.append(7 ** 3) # and the cube of 7 >>> cubes [1, 8, 27, 64, 125, 216, 343]
Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:
>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g'] >>> letters ['a', 'b', 'c', 'd', 'e', 'f', 'g'] >>> # replace some values >>> letters[2:5] = ['C', 'D', 'E'] >>> letters ['a', 'b', 'C', 'D', 'E', 'f', 'g'] >>> # now remove them >>> letters[2:5] =  >>> letters ['a', 'b', 'f', 'g'] >>> # clear the list by replacing all the elements with an empty list >>> letters[:] =  >>> letters 
The built-in function
len() also applies to lists:
>>> letters = ['a', 'b', 'c', 'd'] >>> len(letters) 4
It is possible to nest lists (create lists containing other lists), for example:
>>> a = ['a', 'b', 'c'] >>> n = [1, 2, 3] >>> x = [a, n] >>> x [['a', 'b', 'c'], [1, 2, 3]] >>> x ['a', 'b', 'c'] >>> x 'b'
3.2. First Steps Towards Programming¶
Of course, we can use Python for more complicated tasks than adding two and two together. For instance, we can write an initial sub-sequence of the Fibonacci series as follows:
>>> # Fibonacci series: ... # the sum of two elements defines the next ... a, b = 0, 1 >>> while b < 10: ... print b ... a, b = b, a+b ... 1 1 2 3 5 8
This example introduces several new features.
The first line contains a multiple assignment: the variables
bsimultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.
whileloop executes as long as the condition (here:
b < 10) remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C:
<=(less than or equal to),
>=(greater than or equal to) and
!=(not equal to).
The body of the loop is indented: indentation is Python’s way of grouping statements. At the interactive prompt, you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; all decent text editors have an auto-indent facility. When a compound statement is entered interactively, it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line). Note that each line within a basic block must be indented by the same amount.
>>> i = 256*256 >>> print 'The value of i is', i The value of i is 65536
A trailing comma avoids the newline after the output:
>>> a, b = 0, 1 >>> while b < 1000: ... print b, ... a, b = b, a+b ... 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
Note that the interpreter inserts a newline before it prints the next prompt if the last line was not completed.
|||Unlike other languages, special characters such as |