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3.1.1.Numbers¶

The interpreter acts as a simple calculator: you can type an expression at itand it will write the value. Expression syntax is straightforward: theoperators+,-,* and/ work just like in most other languages(for example, Pascal or C); parentheses (()) can be used for grouping.For example:

>>>2+24>>>50-5*620>>>(50-5.0*6)/45.0>>>8/5.01.6

The integer numbers (e.g.2,4,20) have typeint,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. Ifboth operands are of typeint,floor division is performedand anint is returned. If either operand is afloat,classic division is performed and afloat is returned. The//operator is also provided for doing floor division no matter what theoperands are. The remainder can be calculated with the% operator:

>>>17/3# int / int -> int5>>>17/3.0# int / float -> float5.666666666666667>>>17//3.0# explicit floor division discards the fractional part5.0>>>17%3# the % operator returns the remainder of the division2>>>5*3+2# result * divisor + remainder17

With Python, it is possible to use the** operator to calculate powers1:

>>>5**2# 5 squared25>>>2**7# 2 to the power of 7128

The equal sign (=) is used to assign a value to a variable. Afterwards, noresult is displayed before the next interactive prompt:

>>>width=20>>>height=5*9>>>width*height900

If a variable is not “defined” (assigned a value), trying to use it willgive you an error:

>>>n# try to access an undefined variableTraceback (most recent call last):  File"<stdin>", line1, in<module>NameError:name 'n' is not defined

There is full support for floating point; operators with mixed type operandsconvert the integer operand to floating point:

>>>3*3.75/1.57.5>>>7.0/23.5

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 issomewhat easier to continue calculations, for example:

>>>tax=12.5/100>>>price=100.50>>>price*tax12.5625>>>price+_113.0625>>>round(_,2)113.06

This variable should be treated as read-only by the user. Don’t explicitlyassign a value to it — you would create an independent local variable with thesame name masking the built-in variable with its magic behavior.

In addition toint andfloat, Python supports other types ofnumbers, such asDecimal andFraction.Python also has built-in support forcomplex numbers,and uses thej orJ suffix to indicate the imaginary part(e.g.3+5j).

3.1.2.Strings¶

Besides numbers, Python can also manipulate strings, which can be expressedin several ways. They can be enclosed in single quotes ('...') ordouble quotes ("...") with the same result2.\ can be usedto 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 andspecial characters are escaped with backslashes. While this might sometimeslook different from the input (the enclosing quotes could change), the twostrings are equivalent. The string is enclosed in double quotes ifthe string contains a single quote and no double quotes, otherwise it isenclosed in single quotes. Theprint statement produces a morereadable output, by omitting the enclosing quotes and by printing escapedand 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.'>>>prints# with print, \n produces a new lineFirst line.Second line.

If you don’t want characters prefaced by\ to be interpreted asspecial characters, you can useraw strings by adding anr beforethe first quote:

>>>print'C:\some\name'# here \n means newline!C:\someame>>>printr'C:\some\name'# note the r before the quoteC:\some\name

String literals can span multiple lines. One way is using triple-quotes:"""...""" or'''...'''. End of lines are automaticallyincluded in the string, but it’s possible to prevent this by adding a\ atthe 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, andrepeated with*:

>>># 3 times 'un', followed by 'ium'>>>3*'un'+'ium''unununium'

Two or morestring literals (i.e. the ones enclosed between quotes) nextto each other are automatically concatenated.

>>>'Py''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.'

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 beindexed (subscripted), with the first character having index 0.There is no separate character type; a character is simply a string of sizeone:

>>>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 usedto 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. Thismakes sure thats[:i]+s[i:] is always equal tos:

>>>word[:2]+word[2:]'Python'>>>word[:4]+word[4:]'Python'

Slice indices have useful defaults; an omitted first index defaults to zero, anomitted 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 pointingbetween characters, with the left edge of the first character numbered 0.Then the right edge of the last character of a string ofn characters hasindexn, for example:

+---+---+---+---+---+---+|P|y|t|h|o|n|+---+---+---+---+---+---+0123456-6-5-4-3-2-1

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 fromi toj consists of all characters between the edges labeledi andj,respectively.

For non-negative indices, the length of a slice is the difference of theindices, if both are within bounds. For example, the length ofword[1:3] is2.

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

>>>word[42]# the word only has 6 charactersTraceback (most recent call last):  File"<stdin>", line1, in<module>IndexError:string index out of range

However, out of range slice indexes are handled gracefully when used forslicing:

>>>word[4:42]'on'>>>word[42:]''

Python strings cannot be changed — they areimmutable.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 functionlen() 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 tothe programmer: the Unicode object. It can be used to store and manipulateUnicode data (seehttp://www.unicode.org/) and integrates well with the existingstring objects, providing auto-conversions where necessary.

Unicode has the advantage of providing one ordinal for every character in everyscript used in modern and ancient texts. Previously, there were only 256possible ordinals for script characters. Texts were typically bound to a codepage which mapped the ordinals to script characters. This lead to very muchconfusion especially with respect to internationalization (usually written asi18n —'i' + 18 characters +'n') of software. Unicode solvesthese problems by defining one code page for all scripts.

Creating Unicode strings in Python is just as simple as creating normalstrings:

>>>u'Hello World !'u'Hello World !'

The small'u' in front of the quote indicates that a Unicode string issupposed to be created. If you want to include special characters in the string,you can do so by using the PythonUnicode-Escape encoding. The followingexample shows how:

>>>u'Hello\u0020World !'u'Hello World !'

The escape sequence\u0020 indicates to insert the Unicode character withthe ordinal value 0x0020 (the space character) at the given position.

Other characters are interpreted by using their respective ordinal valuesdirectly as Unicode ordinals. If you have literal strings in the standardLatin-1 encoding that is used in many Western countries, you will find itconvenient that the lower 256 characters of Unicode are the same as the 256characters of Latin-1.

For experts, there is also a raw mode just like the one for normal strings. Youhave to prefix the opening quote with ‘ur’ to have Python use theRaw-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 canbe necessary in regular expressions.

Apart from these standard encodings, Python provides a whole set of other waysof creating Unicode strings on the basis of a known encoding.

The built-in functionunicode() provides access to all registered Unicodecodecs (COders and DECoders). Some of the more well known encodings which thesecodecs can convert areLatin-1,ASCII,UTF-8, andUTF-16. The latter twoare variable-length encodings that store each Unicode character in one or morebytes. The default encoding is normally set to ASCII, which passes throughcharacters 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"<stdin>", line1, 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 anencode() method that takes one argument, thename 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 correspondingUnicode string from it, you can use theunicode() function with theencoding 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 ofcompound data types, used to group together othervalues. The most versatile is thelist, which can be written as a list ofcomma-separated values (items) between square brackets. Lists might containitems 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-insequence type), lists can beindexed and sliced:

>>>squares[0]# indexing returns the item1>>>squares[-1]25>>>squares[-3:]# slicing returns a new list[9, 16, 25]

All slice operations return a new list containing the requested elements. Thismeans 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 areimmutable, lists are amutabletype, 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 usingtheappend()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 thelist 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 functionlen() also applies to lists:

>>>letters=['a','b','c','d']>>>len(letters)4

It is possible to nest lists (create lists containing other lists), forexample:

>>>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'