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3. An Informal Introduction to Python¶

In the following examples, input and output are distinguished by the presence orabsence of prompts (>>> and …): to repeat the example, you must typeeverything after the prompt, when the prompt appears; lines that do not beginwith a prompt are output from the interpreter. Note that a secondary prompt on aline by itself in an example means you must type a blank line; this is used toend a multi-line command.

You can toggle the display of prompts and output by clicking on >>>in the upper-right corner of an example box.If you hide the promptsand output for an example, then you can easily copy and paste the inputlines into your interpreter.

Many of the examples in this manual, even those entered at the interactiveprompt, include comments.Comments in Python start with the hash character,#, and extend to the end of the physical line.A comment may appear at thestart of a line or following whitespace or code, but not within a stringliteral.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 maybe omitted when typing in examples.

Some examples:

# this is the first commentspam = 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 theprimary prompt, >>>.(It shouldn’t take long.)

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*6) / 45.0>>> 8 / 5# division always returns a floating point number1.6

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.

Division (/) always returns a float.To do floor division andget an integer result (discarding any fractional result) you can use the //operator; to calculate the remainder you can use %:

>>> 17 / 3# classic division returns a float5.666666666666667>>>>>> 17 // 3# floor division discards the fractional part5>>> 17 % 3# the % operator returns the remainder of the division2>>> 5 * 3 + 2# floored quotient * divisor + remainder17

With Python, it is possible to use the ** operator to calculate powers 1:

>>> 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 "", line 1, in NameError: name 'n' is not defined

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

>>> 4 * 3.75 - 114.0

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 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 to int and float, Python supports other types ofnumbers, 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 expressedin several ways.They can be enclosed in single quotes ('...') ordouble quotes ("...") with the same result 2.\ 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.The print() function 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.'>>> print(s)# 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 use raw strings by adding an r beforethe first quote:

>>> print('C:\some\name')# here \n means newline!C:\someame>>> print(r'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] -hDisplay this usage message -H hostname Hostname to connect to""")

produces the following output (note that the initial newline is not included):

Usage: thingy [OPTIONS] -hDisplay 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 more string 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 literalFile "", line 1prefix 'thon'^SyntaxError: invalid syntax>>> ('un' * 3) 'ium'File "", line 1('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 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 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'

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'

Note how the start is always included, and the end always excluded.Thismakes sure that s[:i] + s[i:] is always equal to s:

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

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 of n characters hasindex 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 toj 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 theindices, if both are within bounds.For example, the length of word[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 "", line 1, in 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 are immutable.Therefore, assigning to an indexed position in the string results in an error:

>>> word[0] = 'J'Traceback (most recent call last):File "", line 1, in TypeError: 'str' object does not support item assignment>>> word[2:] = 'py'Traceback (most recent call last):File "", line 1, in 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

See also

Text Sequence Type — str

Strings are examples of sequence types, and support the commonoperations supported by such types.

String Methods

Strings support a large number of methods forbasic transformations and searching.

Formatted string literals

String literals that have embedded expressions.

Format String Syntax

Information about string formatting with str.format().

printf-style String Formatting

The old formatting operations invoked when strings arethe left operand of the % operator are described in more detail here.

3.1.3. Lists¶

Python knows a number of compound data types, used to group together othervalues.The most versatile is the list, 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-in sequence types), 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 ashallow copy of the list:

>>> squares[:][1, 4, 9, 16, 25]

Lists also support 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 usingthe 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 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 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), 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'3.2. First Steps Towards Programming¶

Of course, we can use Python for more complicated tasks than adding two and twotogether.For instance, we can write an initial sub-sequence of theFibonacci seriesas follows:

>>> # Fibonacci series:... # the sum of two elements defines the next... a, b = 0, 1>>> while a > i = 256*256>>> print('The value of i is', i)The value of i is 65536

The keyword argument end can be used to avoid the newline after the output,or end the output with a different string:

>>> a, b = 0, 1>>> while a

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