## Monday, September 19, 2011

### Python - An Introduction

In Python, input and output are distinguished by the presence or absence of prompts (>>> and ...). You must type everything after the prompt, when the prompt appears; lines that do not begin with a prompt are output from the interpreter. The comments in Python starts with an hash(#) character and ends when the physical line ends.
# this is the first comment
SPAM = 1                 # and this is the second comment
# ... and now a third!
STRING = "# This is not a comment."

Using Python as calculator

Just like Ruby language (introduced previously) python can also be used as a calculator.
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
>>> # This is a comment
... 2+2
4
>>> 2+2  # and a comment on the same line as code
4
>>> (50-5*6)/4
5.0
>>> 8/5 # Fractions aren't lost when dividing integers
1.6

To do integer division and get an integer result, discarding any fractional result, there is another operator,//:

>>> # Integer division returns the floor:
... 7//3
2
>>> 7//-3
-3

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

A value can be assigned to several variables simultaneously:

>>> x = y = z = 0  # Zero x, y and z
>>> x
0
>>> y
0
>>> z
0

Variables must be “defined” (assigned a value) before they can be used, or an error will occur:

>>> # try to access an undefined variable
... n
Traceback (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 operands convert the integer operand to floating point:

>>> 3 * 3.75 / 1.5
7.5
>>> 7.0 / 2
3.5

Complex numbers are also supported; imaginary numbers are written with a suffix of j or J.  Complex numbers with a nonzero real component are written as (real+imagj), or can be created with the complex(real, imag) function.

>>> 1j * 1J
(-1+0j)
>>> 1j * complex(0, 1)
(-1+0j)
>>> 3+1j*3
(3+3j)
>>> (3+1j)*3
(9+3j)
>>> (1+2j)/(1+1j)
(1.5+0.5j)

Complex numbers are always represented as two floating point numbers, the real and imaginary part.  To extract these parts from a complex number z, use z.real and z.imag.

>>> a=1.5+0.5j
>>> a.real
1.5
>>> a.imag
0.5

The conversion functions to floating point and integer (float(), int()) don’t work for complex numbers — there is not one correct way to convert a complex number to a real number. Use abs(z) to get its magnitude (as a float) or z.real to get its real part:

>>> a=3.0+4.0j
>>> float(a)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: can't convert complex to float; use abs(z)
>>> a.real
3.0
>>> a.imag
4.0
>>> abs(a)  # sqrt(a.real**2 + a.imag**2)
5.0

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(_, 2)
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.

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