At this point, we’ve written code to draw some interesting features in our inflammation data, loop over all our data files to quickly draw these plots for each of them, and have Python make decisions based on what it sees in our data. But, our code is getting pretty long and complicated; what if we had thousands of datasets, and didn’t want to generate a figure for every single one? Commenting out the figure-drawing code is a nuisance. Also, what if we want to use that code again, on a different dataset or at a different point in our program? Cutting and pasting it is going to make our code get very long and very repetitive, very quickly. We’d like a way to package our code so that it is easier to reuse, and Python provides for this by letting us define things called ‘functions’ - a shorthand way of re-executing longer pieces of code.
Let’s start by defining a function
kelvin_to_celsius that converts temperatures from Kelvin to Celsius:
# Let's get our import statements out of the way first
The function definition opens with the word
def, which is followed by the name of the function and a parenthesized list of parameter names. The body of the function — the statements that are executed when it runs — is indented below the definition line, typically by four spaces.
When we call the function, the values we pass to it are assigned to those variables so that we can use them inside the function. Inside the function, we use a return statement to send a result back to whoever asked for it.
Let’s try running our function. Calling our own function is no different from calling any other function:
We’ve successfully called the function that we defined, and we have access to the value that we returned.
We are using Python 3 division, which always returns a floating point number:
Unfortunately, this wasn’t the case in Python 2:
If you are using Python 2 and want to keep the fractional part of division you need to convert one or the other number to floating point:
And if you want an integer result from division in Python 3, use a double-slash:
Now that we’ve seen how to turn Kelvin into Celsius, let's try converting Celsius to Fahrenheit:
What about converting Kelvin to Fahrenheit? We could write out the formula, but we don’t need to. Instead, we can compose the two functions we have already created:
This is our first taste of how larger programs are built: we define basic operations, then combine them in ever-larger chunks to get the effect we want. Real-life functions will usually be larger than the ones shown here — typically half a dozen to a few dozen lines — but they shouldn’t ever be much longer than that, or the next person who reads it won’t be able to understand what’s going on.
Now that we know how to wrap bits of code up in functions, we can make our inflammation analyasis easier to read and easier to reuse. First, let’s make an
analyse function that generates our plots:
and another function called
detect_problems that checks for those systematics we noticed:
Notice that rather than jumbling this code together in one giant
for loop, we can now read and reuse both ideas separately. We can reproduce the previous analysis with a much simpler
# First redefine our list of filenames from the last lesson
By giving our functions human-readable names, we can more easily read and understand what is happening in the
for loop. Even better, if at some later date we want to use either of those pieces of code again, we can do so in a single line.
Once we start putting things in functions so that we can re-use them, we need to start testing that those functions are working correctly. To see how to do this, let’s write a function to center a dataset around a particular value:
We could test this on our actual data, but since we don’t know what the values ought to be, it will be hard to tell if the result was correct. Instead, let’s use NumPy to create a matrix of 0’s and then center that around 3:
That looks right, so let’s try
center on our real data:
It’s hard to tell from the default output whether the result is correct, but there are a few simple tests that will reassure us:
That seems almost right: the original mean was about 6.1, so the lower bound from zero is how about -6.1. The mean of the centered data isn’t quite zero — we’ll explore why not in the challenges — but it’s pretty close. We can even go further and check that the standard deviation hasn’t changed:
Those values look the same, but we probably wouldn’t notice if they were different in the sixth decimal place. Let’s do this instead:
Again, the difference is very small. It’s still possible that our function is wrong, but it seems unlikely enough that we should probably get back to doing our analysis. We have one more task first, though: we should write some documentation for our function to remind ourselves later what it’s for and how to use it.
The usual way to put documentation in software is to add comments like this:
# centre(data, desired): return a new array containing the original data centered around the desired value.
There’s a better way, though. If the first thing in a function is a string that isn’t assigned to a variable, that string is attached to the function as its documentation:
This is better because we can now ask Python’s built-in help system to show us the documentation for the function:
A string like this is called a docstring. We don’t need to use triple quotes when we write one, but if we do, we can break the string across multiple lines:
We have passed parameters to functions in two ways: directly, as in
type(data), and by name, as in
numpy.loadtxt(fname='something.csv', delimiter=','). In fact, we can pass the filename to
loadtxt without the
but we still need to say
To understand what’s going on, and make our own functions easier to use, let’s re-define our center function like this:
The key change is that the second parameter is now written
desired=0.0 instead of just
desired. If we call the function with two arguments, it works as it did before:
But we can also now call it with just one parameter, in which case
desired is automatically assigned the default value of 0.0:
This is handy: if we usually want a function to work one way, but occasionally need it to do something else, we can allow people to pass a parameter when they need to but provide a default to make the normal case easier. The example below shows how Python matches values to parameters:
As this example shows, parameters are matched up from left to right, and any that haven’t been given a value explicitly get their default value. We can override this behavior by naming the value as we pass it in:
With that in hand, let’s look at the help for numpy.loadtxt:
There’s a lot of information here, but the most important part is the first couple of lines:
, comments='#', delimiter=None, converters=None, skiprows=0, usecols=None, unpack=False, ndmin=0)
This tells us that loadtxt has one parameter called fname that doesn’t have a default value, and eight others that do. If we call the function like this:
then the filename is assigned to
fname (which is what we want), but the delimiter string
',' is assigned to
dtype rather than
dtype is the second parameter in the list. However ',' isn’t a known
dtype so our code produced an error message when we tried to run it. When we call
loadtxt we don’t have to provide
fname= for the filename because it’s the first item in the list, but if we want the ',' to be assigned to the variable
delimiter, we do have to provide
delimiter= for the second parameter since
delimiter is not the second parameter in the list.
“Adding” two strings produces their concatenation:
'a' + 'b' is
'ab'. Write a function called
fence that takes two parameters called
wrapper and returns a new string that has the wrapper character at the beginning and end of the original. A call to your function should look like this:
If the variable
s refers to a string, then
s is the string’s first character and
s[-1] is its last. Write a function called
outer that returns a string made up of just the first and last characters of its input. A call to your function should look like this:
What does the following piece of code display when run - and why?
f = 0 k = 0 def f2k(f): k = ((f-32)*(5.0/9.0)) + 273.15 return k f2k(8) f2k(41) f2k(32) print(k)