5. An Informal Introduction to Python¶
[The source material is from Python 3.5.1, but the contents of this tutorial should apply to almost any version of Python 3]
Many of the examples in this manual, even those entered at the
interactive prompt, include comments. Comments in Python start with the
#, 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
5.1. Using Python as a Calculator¶
Let’s try some simple Python commands.
The interpreter acts as a simple calculator: you can type an expression
at it and it will write the value. Expression syntax is straightforward:
/ work just like in most other
languages (for example, Pascal or C); parentheses (
()) can be used
for grouping. For example:
The integer numbers (e.g.
20) have type
the ones with a fractional part (e.g.
1.6) have type
We will see more about numeric types later in the tutorial.
/) always returns a float. To do floor
and get an integer result (discarding any fractional result) you can use
// operator; to calculate the remainder you can use
With Python, it is possible to use the
** operator to calculate
Do note that
** has higher precedence than
-, so if you want a
negative base you will need parentheses:
The equal sign (
=) is used to assign a value to a variable.
Afterwards, no result is displayed before the next interactive prompt:
If a variable is not defined (assigned a value), trying to use it will give you an error:
--------------------------------------------------------------------------- NameError Traceback (most recent call last) <ipython-input-15-3f95c5a59d25> in <module>() ----> 1 n # Try to access an undefined variable. 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:
In interactive mode, the last printed expression is assigned to the
_. This means that when you are using Python as a desk
calculator, it is somewhat easier to continue calculations, for example:
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
float, Python supports other types of
numbers, such as
Python also has built-in support for complex
and uses the
J suffix to indicate the imaginary part (e.g.
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.
be used to escape quotes:
'"Yes," he said.'
'"Yes," he said.'
'"Isn't," she 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
function produces a more readable output, by omitting the enclosing
quotes and by printing escaped and special characters:
'"Isn't," she said.'
"Isn't," she said.
'First line.nSecond 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
the first quote:
String literals can span multiple lines. One way is using triple-quotes:
'''...'''. End of lines are automatically included
in the string, but it’s possible to prevent this by adding a
the end of the line. The following example:
Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to
Strings can be concatenated (glued together) with the
and repeated with
Two or more string literals (i.e. the ones enclosed between quotes) next to each other are automatically concatenated.
This only works with two literals though, not with variables or expressions:
File "<ipython-input-36-6fcf19fbd400>", line 2 prefix 'thon' # Can't concatenate a variable and a string literal. ^ SyntaxError: invalid syntax
File "<ipython-input-37-826b8aeb7d3b>", line 1 ('un' * 3) 'ium' ^ SyntaxError: invalid syntax
If you want to concatenate variables or a variable and a literal, use
This feature is particularly useful when you want to break long strings:
'Put several strings within parentheses to have them joined together.'
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:
Indices may also be negative numbers, to start counting from the right:
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 substring:
Note how the start is always included, and the end always excluded. This
makes sure that
s[:i] + s[i:] is always equal to
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.
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
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:
--------------------------------------------------------------------------- IndexError Traceback (most recent call last) <ipython-input-52-80def1e3ff80> in <module>() ----> 1 word # The word only has 6 characters. IndexError: string index out of range
Python strings cannot be changed, they are immutable. Therefore, assigning to an indexed position in the string results in an error:
--------------------------------------------------------------------------- TypeError Traceback (most recent call last) <ipython-input-55-197b67ffdd83> in <module>() ----> 1 word = 'J' TypeError: 'str' object does not support item assignment
--------------------------------------------------------------------------- TypeError Traceback (most recent call last) <ipython-input-56-0786521edfdd> in <module>() ----> 1 word[2:] = 'py' TypeError: 'str' object does not support item assignment
The built-in function
returns the length of a string:
- Text Sequence Type str: Strings are examples of sequence types, and support the common operations supported by such types.
- String Methods: Strings support a large number of methods for basic transformations and searching.
- Format String
Information about string formatting with
`printf-style String Formatting <https://docs.python.org/3.5/library/stdtypes.html#old-string-formatting>`__: The old formatting operations invoked when strings and Unicode strings are the left operand of the
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.
[1, 4, 9, 16, 25]
Like strings (and all other built-in sequence type), lists can be indexed and sliced:
[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:
[1, 4, 9, 16, 25]
Lists also support operations like concatenation:
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
[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):
[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:
['a', 'b', 'c', 'd', 'e', 'f', 'g']
['a', 'b', 'C', 'D', 'E', 'f', 'g']
['a', 'b', 'f', 'g']
The built-in function
also applies to lists:
It is possible to nest lists (create lists containing other lists), for example:
[['a', 'b', 'c'], [1, 2, 3]]
['a', 'b', 'c']
5.2. 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:
1 1 2 3 5 8
This example introduces several new features.
- The first line contains a multiple assignment: the variables
bsimultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.
`while<https://docs.python.org/3.5/reference/compound_stmts.html#while>`__ loop executes as long as the condition (here:
b < 10) remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C:
<=(less than or equal to),
>=(greater than or equal to) and
!=(not equal to).
- The body of the loop is indented: indentation is Python’s way of grouping statements. At the interactive prompt, you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; all decent text editors have an auto-indent facility. 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). Note that each line within a basic block must be indented by the same amount.
`print()<https://docs.python.org/3.5/library/functions.html#print>`__ function writes the value of the argument(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 arguments, floating point quantities, and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely, like this:
The value of i is 65536