THE PYTHON
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 toggle the display of prompts and output by clicking on >>> in the upper-
right corner of an example box. If you hide the prompts and output for an example,
then you can easily 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 = 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 the
primary prompt, >>>. (It shouldn’t take long.)
3.1.1. Numbers
The interpreter acts as a simple calculator: you can type an expression at it and it will
write the value. Expression syntax is straightforward: the operators +, -, * and / work
just like in most other languages (for example, Pascal or C); parentheses ( ()) can be
used for grouping. For example:
>>>
>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
, >>> # division always returns a floating point number
1.6
The integer numbers (e.g. 2, 4, 20) have type int, the ones with a fractional part
(e.g. 5.0, 1.6) have 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 %:
>>>
>>> # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3 # floor division discards the fractional part
5
>>> 17 % 3 # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2 # floored quotient * divisor + remainder
17
With Python, it is possible to use the ** operator to calculate powers 1:
>>>
>>> 5 ** 2 # 5 squared
25
>>> 2 ** 7 # 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 = 5 * 9
>>> width * height
900
If a variable is not “defined” (assigned a value), trying to use it will give you an error:
>>>
>>> n # try to access an undefined variable
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined
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 toggle the display of prompts and output by clicking on >>> in the upper-
right corner of an example box. If you hide the prompts and output for an example,
then you can easily 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 = 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 the
primary prompt, >>>. (It shouldn’t take long.)
3.1.1. Numbers
The interpreter acts as a simple calculator: you can type an expression at it and it will
write the value. Expression syntax is straightforward: the operators +, -, * and / work
just like in most other languages (for example, Pascal or C); parentheses ( ()) can be
used for grouping. For example:
>>>
>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
, >>> # division always returns a floating point number
1.6
The integer numbers (e.g. 2, 4, 20) have type int, the ones with a fractional part
(e.g. 5.0, 1.6) have 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 %:
>>>
>>> # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3 # floor division discards the fractional part
5
>>> 17 % 3 # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2 # floored quotient * divisor + remainder
17
With Python, it is possible to use the ** operator to calculate powers 1:
>>>
>>> 5 ** 2 # 5 squared
25
>>> 2 ** 7 # 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 = 5 * 9
>>> width * height
900
If a variable is not “defined” (assigned a value), trying to use it will give you an error:
>>>
>>> n # try to access an undefined variable
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined
