3. An Informal Introduction to Python (2024)

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 / can be used to performarithmetic; 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 you can use the // operator; to calculatethe 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 "<stdin>", line 1, 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:

>>> 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. Text¶

Python can manipulate text (represented by type str, so-called“strings”) as well as numbers. This includes characters “!”, words“rabbit”, names “Paris”, sentences “Got your back.”, etc.“Yay! :)”. They can be enclosed in single quotes ('...') or doublequotes ("...") with the same result [2].

>>> 'spam eggs' # single quotes'spam eggs'>>> "Paris rabbit got your back :)! Yay!" # double quotes'Paris rabbit got your back :)! Yay!'>>> '1975' # digits and numerals enclosed in quotes are also strings'1975'

To quote a quote, we need to “escape” it, by preceding it with \.Alternatively, we can use the other type of quotation marks:

>>> '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 Python shell, the string definition and output string can lookdifferent. The print() function produces a more readable output, byomitting the enclosing quotes and by printing escaped and special characters:

>>> s = 'First line.\nSecond line.' # \n means newline>>> s # without print(), special characters are included in the string'First line.\nSecond line.'>>> print(s) # with print(), special characters are interpreted, so \n produces 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

There is one subtle aspect to raw strings: a raw string may not end inan odd number of \ characters; seethe FAQ entry for more informationand workarounds.

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 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 literal File "<stdin>", line 1 prefix 'thon' ^^^^^^SyntaxError: invalid syntax>>> ('un' * 3) 'ium' File "<stdin>", 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 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'

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 "<stdin>", line 1, 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 are immutable.Therefore, assigning to an indexed position in the string results in an error:

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

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 list.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]

Simple assignment in Python never copies data. When you assign a listto a variable, the variable refers to the existing list.Any changes you make to the list through one variable will be seenthrough all other variables that refer to it.:

>>> rgb = ["Red", "Green", "Blue"]>>> rgba = rgb>>> id(rgb) == id(rgba) # they reference the same objectTrue>>> rgba.append("Alph")>>> rgb["Red", "Green", "Blue", "Alph"]

All slice operations return a new list containing the requested elements. Thismeans that the following slice returns ashallow copy of the list:

>>> correct_rgba = rgba[:]>>> correct_rgba[-1] = "Alpha">>> correct_rgba["Red", "Green", "Blue", "Alpha"]>>> rgba["Red", "Green", "Blue", "Alph"]

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'