3. An Informal Introduction to Python — Python 2.7.18 documentation (original) (raw)

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.

3.1. 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.)

3.1.1. 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 50 - 56 20 (50 - 5.06) / 4 5.0 8 / 5.0 1.6

The integer numbers (e.g. 2, 4, 20) have type int, the ones with a fractional part (e.g. 5.0, 1.6) have typefloat. We will see more about numeric types later in the tutorial.

The return type of a division (/) operation depends on its operands. If both operands are of type int, floor division is performed and an int is returned. If either operand is a float, 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 % operator:

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 1:

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 "", line 1, in 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 int and float, Python supports other types of numbers, such as Decimal and Fraction. Python also has built-in support for complex numbers, and uses the j or J suffix to indicate the imaginary part (e.g. 3+5j).

3.1.2. 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 ("...") with the same result 2. \ 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," they said.' '"Yes," they said.' ""Yes," they said." '"Yes," they said.' '"Isn't," they said.' '"Isn't," they 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 print statement produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters:

'"Isn't," they said.' '"Isn't," they said.' print '"Isn't," they said.' "Isn't," they 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 r before 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:"""...""" or '''...'''. End of lines are automatically included in the string, but it’s possible to prevent this by adding a \ at 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 repeated with *:

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.

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.'

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'

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[0] # character in position 0 'P' word[5] # 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 s:

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 word[1:3] is 2.

Attempting to use an index that is too large will result in an error:

word[42] # the word only has 6 characters Traceback (most recent call last): File "", line 1, in 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[0] = '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

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 asi18n — '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 \uXXXXconversion 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 withstr(), 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 "", 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'

3.1.4. 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. 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[0] # 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]

Unlike strings, which are immutable, lists are a mutabletype, i.e. it is possible to change their content:

cubes = [1, 8, 27, 65, 125] # something's wrong here 4 ** 3 # the cube of 4 is 64, not 65! 64 cubes[3] = 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 the 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[0] ['a', 'b', 'c'] x[0][1] '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.

Footnotes

1

Since ** has higher precedence than -, -3**2 will be interpreted as -(3**2) and thus result in -9. To avoid this and get 9, you can use (-3)**2.

2

Unlike other languages, special characters such as \n have the same meaning with both single ('...') and double ("...") quotes. The only difference between the two is that within single quotes you don’t need to escape " (but you have to escape \') and vice versa.