An Informal Introduction to Python

COMMENT: Original Source

In the following examples, the “>>>” and “…” prompts you see at the start of a line are printed by the interpreter - you do not have to type those! To repeat the example, type in everything you see after the prompt. The “…” 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.

Any line not starting with a prompt is the output of the interpreter.

Many of the examples in this manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash (also knows as pound) character, “#”. Once started, comments extend to the end of the 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, “>>> “.

Numbers #

The interpreter acts as a simple calculator: you type in expressions, and it echoes back the resulting value. Python’s expression syntax is straightforward: the operators +, -, * and / work just like in most other programming languages (for example, C or Java), and 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 rounds down
... 7/3
2
>>> 7/-3
-3

The equal sign (“=”) is used to assign a value to a variable. In this case, the value isn’t echoed back, but you can type in the name of a variable to see its value:

>>> width = 20
>>> width
20
>>> height = 5*9
>>> width * height
900

A value can be assigned to several variables simultaneously:

>>> x = y = z = 0  # Zero is assigned to 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

The interpreter prints up to seventeen significant digits when it prints a floating point number, which can cause surprises when you print out decimal values that cannot be exactly represented by the internal binary representation. For example, the decimal value 0.1 ends up with more decimals than one would expect:

>>> 1.0 / 10.0
0.10000000000000001

For more background, see Appendix B, Floating Point Arithmetic: Issues and Limitations.

Complex numbers are a somewhat advanced mathematical concept, and Python supports them. 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.

>>> 1j * 1J
(-1+0j)
>>> 1j * complex(0,1)
(-1+0j)
>>> 3+1j*3
(3+3j)
>>> (3+1j)*3
(9+3j)
>>> (1+2j)/(1+1j)
(1.5+0.5j)

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 automatically assigned to the variable _, behind the scenes. This makes it easy to reuse the last result when you want to continue a calculation, for example:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06
>>>

This interpreter-specific variable should be treated as read-only by the user. If you assign to it, you will create an independent local variable with the same name, which masks 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:

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 the programming language Icon, substrings can be specified with the slice notation: two indices separated by a colon.

>>> word[4]
'A'
>>> word[0:2]
'He'
>>> word[2:4]
'lp'

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]    # The first two characters
'He'
>>> word[2:]    # Everything except the first two characters
'lpA'
>>> word[:]     # The entire string
'HelpA'

Unlike a C string, Python strings cannot be changed. Assigning to an indexed position in the string results in an error:

>>> word[0] = 'x'
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object doesn't support item assignment
>>> word[:1] = 'Splat'
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object doesn't support slice assignment

However, creating a new string with the combined content is easy and efficient:

>>> 'x' + word[1:]
'xelpA'
>>> 'Splat' + word[4]
'SplatA'

Here’s a useful invariant of slice operations: s[:i] + s[i:] equals s.

>>> word[:2] + word[2:]
'HelpA'
>>> word[:3] + word[3:]
'HelpA'

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.

>>> word[1:100]
'elpA'
>>> word[10:]
''
>>> word[2:1]
''

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:

>>> len('Hello')
5
>>> len('Supercalifragilisticexpialidocious')
34

See Also:

Sequence Types 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.

String Formatting Operations The formatting operations invoked when strings and Unicode strings are the left operand of the % operator are described in more detail here.

Unicode Strings #

The standard string type stores bytes, which often makes it hard to work with non-ASCII text. To address this, Python provides a second string type, the Unicode string, which can be used to store and manipulate Unicode data (see http://www.unicode.org).

Instead of bytes, the Unicode string stores characters from the Unicode character set, which contains distinct codes for all characters in every script used in modern and ancient texts. This allows you to mix texts written in different alphabets freely, without having to keep track of what encoding (or code page) each part belongs to.

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 an 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. Most standard escapes can also be used, such as \n, \x20, etc.

For non-ASCII characters, Python uses the coding directive to map from source code characters to Unicode characters. For example, if the file is marked as iso-8859-1, a byte with the value 177 will be interpreted as a plus/minus sign (±).

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’.

>>> 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.

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'

Lists #

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 replace 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:

The built-in function len also works for lists:

>>> len(a)
8

Lists can be nested:

>>> 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, the following code prints the first few numbers from the mathematical Fibonacci series:

>>> # 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.

Multiple Assignment

The first line contains a multiple assignment statement: 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 of the “=” sign are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.

Loops

The while loop executes as long as the condition (here: `b < 10`) remains true. Python supports several comparison operators, including < (less than), > (greater than), == (equal to), <= (less than or equal to), >= (greater than or equal to) and != (not equal to). You don’t have to use an operator; any non-zero numeric value is true, and zero is false. The condition can also be a string or list value, in fact any sequence; anything with a non-zero length is true, and empty sequences are false.

Grouping Statements through Indentation

The body of the loop is indented: indentation is Python’s way of grouping statements. You can indent using tabs or spaces, as long as all statements that belong to the same group uses the same indentation. In the interpreter, you usually have to type the tabs or spaces yourself, but most text editors have an auto-indent facility that can do this for you. Also, 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).

The print statement

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.