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.
Some examples:
# this is the first comment
SPAM = 1 # and this is the second comment
# ... and now a third!
STRING = "# This is not a comment."
Using Python as a Calculator
Let's try some simple Python commands. Start the interpreter and wait for the
primary prompt, ">>> ". (It shouldn't take long.)
Numbers
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 operators
+, -, * and / work just like
in most other languages (for example, Pascal or C); parentheses can be used for
grouping. For example:
>>> 2+2
4
>>> # This is a comment
... 2+2
4
>>> 2+2 # and a comment on the same line as code
4
>>> (50-5*6)/4
5
>>> # Integer division returns the floor:
... 7/3
2
>>> 7/-3
-3
The equal sign ("=") is used to assign a value to
a variable. Afterwards, no result is displayed before the next interactive
prompt:
A value can be assigned to several variables simultaneously:
>>> x = y = z = 0 # Zero x, y and z
>>> x
0
>>> y
0
>>> z
0
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
Complex numbers are also supported; imaginary numbers are written with a
suffix of "j" or "J". Complex
numbers with a nonzero real component are written as "(real+imagj)",
or can be created with the "complex(real, imag)"
function.
Complex numbers are always represented as two floating point numbers, the
real and imaginary part. To extract these parts from a complex number z,
use z.real and z.imag.
>>> a=1.5+0.5j
>>> a.real
1.5
>>> a.imag
0.5
The conversion functions to floating point and integer (float(),
int() and long()) don't work
for complex numbers -- there is no one correct way to convert a complex number
to a real number. Use abs(z) to get its magnitude (as a
float) or z.real to get its real part.
>>> a=3.0+4.0j
>>> float(a)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: can't convert complex to float; use abs(z)
>>> a.real
3.0
>>> a.imag
4.0
>>> abs(a) # sqrt(a.real**2 + a.imag**2)
5.0
>>>
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:
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.
Strings
Besides numbers, Python can also manipulate strings, which can be expressed
in several ways. They can be enclosed in single quotes or double quotes:
>>> 'spam eggs'
'spam eggs'
>>> 'doesn\'t'
"doesn't"
>>> "doesn't"
"doesn't"
>>> '"Yes," he said.'
'"Yes," he said.'
>>> "\"Yes,\" he said."
'"Yes," he said.'
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
String literals can span multiple lines in several ways. Continuation lines
can be used, with a backslash as the last character on the line indicating that
the next line is a logical continuation of the line:
hello = "This is a rather long string containing\n\
several lines of text just as you would do in C.\n\
Note that whitespace at the beginning of the line is\
significant."
print hello
Note that newlines still need to be embedded in the string using \n;
the newline following the trailing backslash is discarded. This example would
print the following:
This is a rather long string containing
several lines of text just as you would do in C.
Note that whitespace at the beginning of the line is significant.
If we make the string literal a ``raw'' string, however, the \n
sequences are not converted to newlines, but the backslash at the end of the
line, and the newline character in the source, are both included in the string
as data. Thus, the example:
hello = r"This is a rather long string containing\n\
several lines of text much as you would do in C."
print hello
would print:
This is a rather long string containing\n\
several lines of text much as you would do in C.
Or, strings can be surrounded in a pair of matching triple-quotes: """
or '''. End of lines do not need to be escaped when using
triple-quotes, but they will be included in the string.
print """
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
"""
produces the following output:
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
The interpreter prints the result of string operations in the same way as
they are typed for input: inside quotes, and with quotes and other funny
characters escaped by backslashes, to show the precise value. The string is
enclosed in double quotes if the string contains a single quote and no double
quotes, else it's enclosed in single quotes. (The print
statement, described later, can be used to write strings without quotes or
escapes.)
Strings can be concatenated (glued together) with the +
operator, and repeated with *:
>>> word = 'Help' + 'A'
>>> word
'HelpA'
>>> '<' + word*5 + '>'
'<HelpAHelpAHelpAHelpAHelpA>'
Two string literals next to each other are automatically concatenated; the
first line above could also have been written "word = 'Help'
'A'"; this only works with two literals, not with arbitrary string
expressions:
>>> 'str' 'ing' # <- This is ok
'string'
>>> 'str'.strip() + 'ing' # <- This is ok
'string'
>>> 'str'.strip() 'ing' # <- This is invalid
File "<stdin>", line 1, in ?
'str'.strip() 'ing'
^
SyntaxError: invalid syntax
Strings can be subscripted (indexed); like in C, the first character of a
string has subscript (index) 0. There is no separate character type; a character
is simply a string of size one. Like in Icon, substrings can be specified with
the slice notation: two indices separated by a colon.
Degenerate slice indices are handled gracefully: an index that is too large
is replaced by the string size, an upper bound smaller than the lower bound
returns an empty string.
Indices may be negative numbers, to start counting from the right. For
example:
>>> word[-1] # The last character
'A'
>>> word[-2] # The last-but-one character
'p'
>>> word[-2:] # The last two characters
'pA'
>>> word[:-2] # Everything except the last two characters
'Hel'
But note that -0 is really the same as 0, so it does not count from the
right!
>>> word[-0] # (since -0 equals 0)
'H'
Out-of-range negative slice indices are truncated, but don't try this for
single-element (non-slice) indices:
>>> word[-100:]
'HelpA'
>>> word[-10] # error
Traceback (most recent call last):
File "<stdin>", line 1, in ?
IndexError: string index out of range
The best 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:
+---+---+---+---+---+
| H | e | l | p | A |
+---+---+---+---+---+
0 1 2 3 4 5
-5 -4 -3 -2 -1
The first row of numbers gives the position of the indices 0...5 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 word[1:3]
is 2.
The built-in function len() returns the length of a
string:
>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34
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 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 "i18n"
-- "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 !'
The small "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 \uXXXX
conversion if there is an uneven number of backslashes in front of the small
'u'.
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"���"
u'\xe4\xf6\xfc'
>>> str(u"���")
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.
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.
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. List items need
not all have the same type.
>>> a = ['spam', 'eggs', 100, 1234]
>>> a
['spam', 'eggs', 100, 1234]
Like string indices, list indices start at 0, and lists can be sliced,
concatenated and so on:
Unlike strings, which are immutable, it is possible to change
individual elements of a list:
>>> a
['spam', 'eggs', 100, 1234]
>>> a[2] = a[2] + 23
>>> a
['spam', 'eggs', 123, 1234]
Assignment to slices is also possible, and this can even change the size of
the list or clear it entirely:
>>> # Replace some items:
... a[0:2] = [1, 12]
>>> a
[1, 12, 123, 1234]
>>> # Remove some:
... a[0:2] = []
>>> a
[123, 1234]
>>> # Insert some:
... a[1:1] = ['bletch', 'xyzzy']
>>> a
[123, 'bletch', 'xyzzy', 1234]
>>> # Insert (a copy of) itself at the beginning
>>> a[:0] = a
>>> a
[123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]
>>> # Clear the list: replace all items with an empty list
>>> a[:] = []
>>> a
[]
The built-in function len() also applies to lists:
>>> len(a)
8
It is possible to nest lists (create lists containing other lists), for
example:
>>> q = [2, 3]
>>> p = [1, q, 4]
>>> len(p)
3
>>> p[1]
[2, 3]
>>> p[1][0]
2
>>> p[1].append('xtra') # See section 5.1
>>> p
[1, [2, 3, 'xtra'], 4]
>>> q
[2, 3, 'xtra']
Note that in the last example, p[1] and q really
refer to the same object! We'll come back to object semantics later.
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
a and b simultaneously 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.
The while loop 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), > (greater than), ==
(equal to), <= (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. Python does not (yet!) provide an
intelligent input line editing facility, so 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; most 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.
The print statement writes the value of the
expression(s) it is given. It differs from just writing the expression you
want to write (as we did earlier in the calculator examples) in the way it
handles multiple expressions and strings. Strings are printed without
quotes, and a space is inserted between items, so you can format things
nicely, like this:
>>> 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.
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