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 An Introduction to Python by Guido van Rossum and Fred L. Drake, Jr.Paperback (6"x9"), 124 pagesISBN 0954161769RRP £12.95 (\$19.95)Sales of this book support the Python Software Foundation! Get a printed copy>>>

### 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
>>> # This is a comment
... 2+2
4
>>> 2+2  # and a comment on the same line as code
4
>>> (50-5*6)/4
5
>>> # 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
```

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()` and `long()`) don't work for complex numbers--there is no 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 "<stdin>", 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.

 ISBN 0954161769 An Introduction to Python See the print edition