## 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 (e.g., 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
    >>>

Like in C, the equal sign (`=`) is used to assign a value to a variable. The value of an assignment is not written:

    >>> 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:

    >>> 4 * 2.5 / 3.3
    3.0303030303
    >>> 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=1.5+0.5j
    >>> float(a)
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
    TypeError: can't convert complex to float; use e.g. abs(z)
    >>> a.real
    1.5
    >>> abs(a)
    1.58113883008

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 = 17.5 / 100
    >>> price = 3.50
    >>> price * tax
    0.6125
    >>> price + _
    4.1125
    >>> round(_, 2)
    4.11

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.