One of the strengths of Python is that there are many built-in add-ons - or modules - which contain existing functions, classes, and variables which allow you to do complex tasks in only a few lines of code. In addition, there are many other third-party modules (e.g. Numpy, Scipy, Matplotlib) that can be installed, and you can also develop your own modules that include functionalities you commonly use.
The built-in modules are referred to as the Standard Library, and you can find a full list of the available functionality in the Python Documentation.
To use modules in your Python session or script, you need to import them. The
following example shows how to import the built-in math
module, which
contains a number of useful mathematical functions:
import math
You can then access functions and other objects in the module with math.<function>
, for example:
math.sin(2.3)
math.factorial(20)
math.pi
Because these modules exist, it means that if what you want to do is very common, it means it probably already exists, and you won't need to write it (making your code easier to read).
For example, the numpy
module, which we will talk about next week, contains useful functions for finding e.g. the mean, median, and standard deviation of a sequence of numbers:
import numpy as np
li = [1,2,7,3,1,3]
np.mean(li)
np.median(li)
np.std(li)
Notice that in the above case, we used:
import numpy as np
instead of:
import numpy
which shows that we can rename the module so that it's not as long to type in the program.
Finally, it's also possible to simply import the functions needed directly:
from math import sin, cos
sin(3.4)
cos(3.4)
You may find examples on the internet that use e.g.
from module import *
but this is not recommended, because it will make it difficult to debug programs, since common debugging tools that rely on just looking at the programs will not know all the functions that are being imported.
How do you know which modules exist in the first place? The Python documentation contains a list of modules in the Standard Library, but you can also simply search the web. Once you have a module that you think should contain the right kind of function, you can either look at the documentation for that module, or you can use the tab-completion in IPython:
In [2]: math.<TAB>
math.acos math.degrees math.fsum math.pi
math.acosh math.e math.gamma math.pow
math.asin math.erf math.hypot math.radians
math.asinh math.erfc math.isinf math.sin
math.atan math.exp math.isnan math.sinh
math.atan2 math.expm1 math.ldexp math.sqrt
math.atanh math.fabs math.lgamma math.tan
math.ceil math.factorial math.log math.tanh
math.copysign math.floor math.log10 math.trunc
math.cos math.fmod math.log1p
math.cosh math.frexp math.modf
Does the math.cos
funtion take radians or degrees? Are there functions that can convert between radians and degrees? Use these to find the cosine of 60 degrees, and the sine of pi/6 radians.
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