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.