Beginning Python - From Novice to Professional

18 CHAPTER 1 ■ INSTANT HACKING: THE BASICS
■Note Similar functions exist to convert to other types (for example, long and float). In fact, these aren’t
completely normal functions—they’re type objects. I’ll have more to say about types later. The opposite of
floor is ceil (short for “ceiling”), which finds the smallest integral value larger than or equal to the given
number.
If you are sure that you won’t import more than one function with a given name (from
different modules), you might not want to write the module name each time you call the function.
Then you can use a variant of the import command:
>>> from math import sqrt
>>> sqrt(9)
3.0
After using from module import function, you can use the function without its module prefix.
■Tip You may, in fact, use variables to refer to functions (and most other things in Python). For example, by
performing the assignment foo = math.sqrt you can start using foo to calculate square roots; for example,
foo(4) yields 2.
cmath and Complex Numbers
The sqrt function is used to calculate the square root of a number. Let’s see what happens if we
supply it with a negative number:
>>> from math import sqrt
>>> sqrt(-1)
Traceback (most recent call last):
File "", line 1, in ?
sqrt(-1)
ValueError: math domain error
Well, that’s reasonable. You can’t take the square root of a negative number—or can you?
Of course you can: The square root of a negative number is an imaginary number. (This is a
standard mathematical concept—if you find it a bit too mind-bending, you are free to skip ahead.)
So why couldn’t sqrt deal with it? Because it only deals with floats, and imaginary numbers
(and complex numbers, the sum of real and imaginary numbers) are something completely
different—which is why they are covered by a different module, cmath (for complex math):
>>> import cmath
>>> cmath.sqrt(-1)
1j