3. An Informal Introduction to Python — Python 3.14.4 documentation

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Python
3.14.4 Documentation
The Python Tutorial
3.
An Informal Introduction to Python
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3.
An Informal Introduction to Python
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.
You can use the “Copy” button (it appears in the upper-right corner
when hovering over or tapping a code example), which strips prompts
and omits output, to copy and paste the input lines into your interpreter.
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.
Some examples:
# this is the first comment
spam
# 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 the
primary prompt,
>>>
. (It shouldn’t take long.)
3.1.1.
Numbers
The interpreter acts as a simple calculator: you can type an expression into it
and it will write the value. Expression syntax is straightforward: the
operators
and
can be used to perform
arithmetic; parentheses (
()
) can be used for grouping.
For example:
>>>
>>>
50
20
>>>
50
5.0
>>>
# division always returns a floating-point number
1.6
The integer numbers (e.g.
20
) have type
int
the ones with a fractional part (e.g.
5.0
1.6
) have type
float
. We will see more about numeric types later in the tutorial.
Division (
) always returns a float. To do
floor division
and
get an integer result you can use the
//
operator; to calculate
the remainder you can use
>>>
17
# classic division returns a float
5.666666666666667
>>>
>>>
17
//
# floor division discards the fractional part
>>>
17
# the % operator returns the remainder of the division
>>>
# floored quotient * divisor + remainder
17
With Python, it is possible to use the
**
operator to calculate powers
>>>
**
# 5 squared
25
>>>
**
# 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
>>>
width
height
900
If a variable is not “defined” (assigned a value), trying to use it will
give you an error:
>>>
# try to access an undefined variable
Traceback (most recent call last):
File
""
, line
, 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:
>>>
3.75
14.0
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 is
somewhat easier to continue calculations, for example:
>>>
tax
12.5
100
>>>
price
100.50
>>>
price
tax
12.5625
>>>
price
113.0625
>>>
round
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
or
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 double
quotes (
"..."
) with the same result
>>>
'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 look
different. The
print()
function produces a more readable output, by
omitting the enclosing quotes and by printing escaped and special characters:
>>>
'First line.
\n
Second line.'
# \n means newline
>>>
# without print(), special characters are included in the string
'First line.\nSecond line.'
>>>
# with print(), special characters are interpreted, so \n produces 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
before
the first quote:
>>>
'C:
\t
his
\n
ame'
# here \t means tab, \n means newline
C: his
ame
>>>
'C:\this\name'
# note the r before the quote
C:\this\name
There is one subtle aspect to raw strings: a raw string may not end in
an odd number of
characters; see
the FAQ entry
for more information
and workarounds.
String literals can span multiple lines. One way is using triple-quotes:
"""..."""
or
'''...'''
. End-of-line characters are automatically
included in the string, but it’s possible to prevent this by adding a
at
the end of the line. In the following example, the initial newline is not
included:
>>>
"""
...
Usage: thingy [OPTIONS]
...
-h Display this usage message
...
-H hostname Hostname to connect to
...
"""
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'
>>>
'un'
'ium'
'unununium'
Two or more
string literals
(i.e. the ones enclosed between quotes) next
to 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
""
, line
prefix
'thon'
^^^^^^
SyntaxError
invalid syntax
>>>
'un'
'ium'
File
""
, line
'un'
'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
# character in position 0
'P'
>>>
word
# character in position 5
'n'
Indices may also be negative numbers, to start counting from the right:
>>>
word
# last character
'n'
>>>
word
# second-last character
'o'
>>>
word
'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
# characters from position 0 (included) to 2 (excluded)
'Py'
>>>
word
# characters from position 2 (included) to 5 (excluded)
'tho'
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
[:
# character from the beginning to position 2 (excluded)
'Py'
>>>
word
:]
# characters from position 4 (included) to the end
'on'
>>>
word
:]
# characters from the second-last (included) to the end
'on'
Note how the start is always included, and the end always excluded. This
makes sure that
s[:i]
s[i:]
is always equal to
>>>
word
[:
word
:]
'Python'
>>>
word
[:
word
:]
'Python'
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
characters has
index
, for example:
+---+---+---+---+---+---+
+---+---+---+---+---+---+
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
to
consists of all characters between the edges labeled
and
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
, in

IndexError
string index out of range
However, out of range slice indexes are handled gracefully when used for
slicing:
>>>
word
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
'J'
Traceback (most recent call last):
File
""
, line
, in

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

TypeError
'str' object does not support item assignment
If you need a different string, you should create a new one:
>>>
'J'
word
:]
'Jython'
>>>
word
[:
'py'
'Pypy'
The built-in function
len()
returns the length of a string:
>>>
'supercalifragilisticexpialidocious'
>>>
len
34
See also
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.
f-strings
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 are
the 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 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
16
25
>>>
squares
[1, 4, 9, 16, 25]
Like strings (and all other built-in
sequence
types), lists can be
indexed and sliced:
>>>
squares
# indexing returns the item
>>>
squares
25
>>>
squares
:]
# 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
mutable
type, i.e. it is possible to change their content:
>>>
cubes
27
65
125
# something's wrong here
>>>
**
# the cube of 4 is 64, not 65!
64
>>>
cubes
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
list.append()
method
(we will see more about methods later):
>>>
cubes
append
216
# add the cube of 6
>>>
cubes
append
**
# and the cube of 7
>>>
cubes
[1, 8, 27, 64, 125, 216, 343]
Simple assignment in Python never copies data. When you assign a list
to a variable, the variable refers to the
existing list
Any changes you make to the list through one variable will be seen
through all other variables that refer to it.:
>>>
rgb
"Red"
"Green"
"Blue"
>>>
rgba
rgb
>>>
id
rgb
==
id
rgba
# they reference the same object
True
>>>
rgba
append
"Alph"
>>>
rgb
["Red", "Green", "Blue", "Alph"]
All slice operations return a new list containing the requested elements. This
means that the following slice returns a
shallow copy
of the list:
>>>
correct_rgba
rgba
[:]
>>>
correct_rgba
"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 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
'C'
'D'
'E'
>>>
letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>>
# now remove them
>>>
letters
[]
>>>
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
It is possible to nest lists (create lists containing other lists), for
example:
>>>
'a'
'b'
'c'
>>>
>>>
>>>
[['a', 'b', 'c'], [1, 2, 3]]
>>>
['a', 'b', 'c']
>>>
][
'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
>>>
>>>
while
10
...
...
...
This example introduces several new features.
The first line contains a
multiple assignment
: the variables
and
simultaneously 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.
The
while
loop executes as long as the condition (here:
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),
(greater than),
==
(equal to),
<=
(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.
The
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:
>>>
256
256
>>>
'The value of i is'
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:
>>>
>>>
while
1000
...
end
','
...
...
0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
Footnotes
Since
**
has higher precedence than
-3**2
will be
interpreted as
-(3**2)
and thus result in
-9
. To avoid this
and get
, you can use
(-3)**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.
Table of Contents
3. An Informal Introduction to Python
3.1. Using Python as a Calculator
3.1.1. Numbers
3.1.2. Text
3.1.3. Lists
3.2. First Steps Towards Programming
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Using the Python Interpreter
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Python
3.14.4 Documentation
The Python Tutorial
3.
An Informal Introduction to Python
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2001 Python Software Foundation.
This page is licensed under the Python Software Foundation License Version 2.
Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
See
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for more information.
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