Creating a Reproducible Workflow

Introduction

We're now in the home stretch of the workshop - congratulations! Up to this point, we've talked about how to make your code efficient (good programming practice), accurate (testing), and maintainable (modularization + version control). Now we're going to talk about a final and very important concept known as reproducibility. Victoria Stodden has written extensively about the idea of reproducibility in scientific software - you may want to look up some of her papers for reference.

For our purposes, we can summarize the goal of reproducibility in two related ways, one technical and one colloquial.

In a technical sense, your goal is to have a complete chain of custody (ie, provenance) from your raw data to your finished results and figures. That is, you should always be able to figure out precisely what data and what code were used to generate what result - there should be no "missing links". If you have ever had the experience of coming across a great figure that you made months ago and having no idea how in the world you made it, then you understand why provenance is important. Or, worse, if you've ever been unable to recreate the results that you once showed on a poster or (gasp) published in a paper...

In a colloquial sense, I should be able to sneak into your lab late at night, delete everything except for your raw data and your code, and you should be able to run a single command to regenerate EVERYTHING, including all of your results, tables, and figures in their final, polished form. Think of this as the "push button" workflow. This is your ultimate organizational goal as a computational scientist. Importantly, note that this rules out the idea of manual intervention at any step along the way - no tinkering with figure axes in a pop-up window, no deleting columns from tables, no copying data from one folder to another, etc. All of that needs to be fully automated.

As an added bonus, if you couple this with a version control system that tracks changes over time to your raw data and your code, you will be able to instantly recreate your results from any stage in your research (the lab presentation version, the dissertation version, the manuscript version, the Nobel Prize committee version, etc.). Wouldn't that be nice?

To illustrate these ideas, we'll set up a small but realistic research project that follows a reproducible workflow. Just like you would in your own research projects, we'll go through the following key steps:

  1. Create a clear and useful directory structure for our project.
  2. Set up (and use) Git to track our changes.
  3. Add the raw data to our project.
  4. Write the core "scientific code" to perform the analysis, including tests.
  5. Create a "runall" script to generate results for our specific project.
  6. Push the button and watch the magic.

One final note - the workflow that we're following here is just a suggestion. Organizing code and data is an art, and a room of 100 scientists will give you 101 opinions about how to do it best. Consider the below a useful place to get started, and don't hesitate to tinker and branch out as you get a better feel for this process. You also might want to review William Noble's paper on this topic for more ideas.

1. Setting up the project directory

Let's create a project (a reasonably self-contained set of code, data, and results to answer a discrete scientific question) that will count the animals sighted in two years of camera trap surveys. We begin by creating a directory called camera_analysis in a convenient place on our hard drive. You might want to create a main directory called Projects or Research in your home folder or in your Documents folder to hold the directories for all of your individual research projects.

Now, within the camera_analysis directory, create four subdirectories:

.
|-- data
|-- man
|-- results
|-- src

The data directory will hold all of the raw data associated with the project, which in this case will be just a single large csv file containing data on the animal sightings. The man folder, short for manuscript, will (someday) contain the manuscript that we'll write describing the results of our analysis (you were planning on using version control for your manuscript too, weren't you?). The results folder will contain the results of our analysis, including both tables and figures, and the src directory will contain all of our code.

In a more complex project, each of these directories may have additional subdirectories to help keep things organized.

For bonus points, do this all from the command line.

2. Initialize a Git repository

Since we want to use version control to track the development of our project, we'll start off right away by initializing an empty Git repository within this directory. To do this, open a Terminal window, navigate to the main camera_analysis directory, and run the command git init.

As you add things to the project directory, and modify old things, you'll want to frequently commit your changes as we discussed in the Git tutorial.

3. Add raw data

Often, we start a project with a particular data file, or set of data files. In this case, we have the file sightings_tab_lg.csv, which contains the records that we want to analyze. Download this file here and place it in the data subdirectory.

Now we reach an interesting question - should your data directory be placed under version control (ie, should you git add and git commit these files)? Although you might automatically think that this is necessary, in principle our raw data should never change - that is, there's only one version (the original version!), and it will never be updated. As a result, it's not necessarily useful to place this file under version control for the purpose of tracking changes to it.

A reasonable rule of thumb for getting started is that if the file is realatively small (ours is < 100k), go ahead and commit it to the Git repository, as you won't be wasting much hard disk space. Additionally, the file will then travel with your code, so if you push your repository to Github (for example) and one of your collaborators clones a copy, they'll have everything they need to generate your results.

However, if your file is relatively large AND is backed up elsewhere, you might want to avoid making a duplicate copy in the .git directory.

In either case, you'll want to ensure that every one of your data files has some sort of metadata associated with it to describe where it came from, how it got to you, the meaning of the columns, etc. There are many formats for metadata that vary from simple to very complex. If you're interested in following good ecological best practices, you may want to review the Ecological Metadata Language standards and the tool Morpho for creating metadata files. For your own private work, make sure that, at a minimum, you create a README.txt file that describes your data as best you can.

Copy and paste the text below into a README.txt file and place it in the data subdirectory. Remember that this is a bare-bones description - in your own work, you'll want to include as much information as you have.

Data received via email on April 1, 2013 from Professor Smith. Includes
records from camera trap surveys conducted by John Doe and Jane Doe from
2011-2012. Method of collection, site locations, and additional
descriptions are found in John Doe's dissertation, Chapter 3 Appendix,
filed August 2012 at UC Berkeley.

At this point, your project directory should look like this:

.
|-- data
|   |-- README.txt
|   |-- sightings_tab_lg.csv
|-- man
|-- results
|-- src

Commit both the data and README files to your git repository.

What about the case in which your raw data is hosted elsewhere, on a SQL server, for example, or a shared hard drive with your lab? Now your data is living somewhere else, and you don't necessarily have direct control over its provenance (what if someone changes it while you weren't looking?). In this situation, you should try to make your runall.py script (see below) make a copy of the metadata associated with the dataset (it does have metadata, doesn't it?), which hopefully will include something like a version number and a last-updated date, and store this along with your results. That way you'll at least have some information on the version of the data that was used. If there's no metadata, try to shame your collaborators into creating some. If all else fails, at least record the date on which your analysis was run so that, in principle, you could later try to find out what state the raw data was in on that date. If you're really nervous about the data changing, though, you might want to look into making yourself a local copy.

4. Write code to perform analysis

Now for the real work - writing the code that will perform our analysis. We'd like to generate two outputs. First, we want to make and save a table that contains a column with the names of the four mammalian carnivores found in our data set - Fox, Wolf, Grizzly, and Wolverine - and a second column that contains the total number of records associated with each species. Second, we'd like to create and save a simple histogram that shows this result visually.

Modules

We've already done the work of writing much of this code earlier today, so at this point, we can simply copy and paste the file mean_sightings.py that you created earlier in the testing lesson into your src subdirectory. Or you can download a complete copy of this file here, making sure to delete the "-full" from the end of the file name.

Note that, by itself, the code in our mean_sightings.py file doesn't actually generate any results for us - this file just contains the core functions that we have written to perform our analysis. Later on, we'll write a small runall.py script that imports these core functions and generates our results for this particular project. In small projects, you can combine these two files, but it's good practice to keep your actual analysis code in a separate module where it can easily be tested and reused later on. This separation of files also supports a mental separation of roles - the module is where you write and test the guts of your code (the stuff that "does science") whereas the runall script is just the few lines that take this "science" and applies it to your particular data set to generate your particular results (the stuff that spits out "these results"). If you wanted to perform the same analysis on a different data set or with a different set of input parameters, for example, you should be able to accomplish that by modifying just the runall file that we'll discuss later.

Note, of course, that copying and pasting in a completed module is not the normal workflow for this step. Normally, you'd spend days/weeks/months working in the src directory, writing code and tests, generating results, looking at the results, writing new code and tests, generating new results, etc. This iterative cycle isn't unlike writing a paper - you spew out a draft that's not too bad, then go back and revise it, then spew out some new material, revise that, etc.

Different people have different favorite approaches and tools for this iterative cycle.

One strategy is to simultaneously work on three files at once - a module like mean_sightings.py, a file to test the functions in your module like test_mean_sightings.py, and a third script that calls various functions from your module and runs them so that you can see whether your code is doing what you want it to do. I sometimes call this scratch.py or something like that, and fill up my Terminal window with hundreds of lines of python scratch.py as I modify my code and look at the results that are saved, results that are printed to the Terminal window, and errors that pop up.

Another strategy is to take advantage of the IPython notebook, where you can write your code in individual cells and easily execute each one sequentially, make sure each cell executes properly, and review the values of variables after each step. This can be great and efficient in the event that you really don't have any idea what your final code will look like. The downside here is that, at the end, you'll be left with one enormous notebook file (probably without unit tests), and you'll need to go back at the end to properly modularize your code into functions and separate files, similar to the structure that we're using in this exercise, so that you can have a fully reproducible workflow. Plus you may end up writing your unit tests at the end (you are going to write them, aren't you?) rather than iteratively with your code as you go. All that said, though, this is a great strategy if you think you need to feel your way around for a while.

Documentation

Documentation is an important part of creating a reproducible workflow - rather than simply reproducing results, you can think of documentation as helping you to reproduce the thought process that led you to design your code in a particular way. Being able to call up and recreate the thought process that led to your code is imporant both for "future you", who will eventually come back to this code in other projects, or at a minimum when writing the manuscript for this project, and for other users who might need to apply or maintain your code in the future.

Every programming language has its own guidelines and style for documentation - regardless of what language you're using, I'd suggest Googling something like " style guide", and similar strings, to read about common conventions for that language. Below, we'll briefly discuss documentation in the context of Python modules, but a similar high-level conceptual framework will apply to any language.

In short, I would suggest mentally dividing your documentation into four basic levels, ranging from the smallest to the broadest scale.

1. Line-level comments

At the smallest scale are line-by-line comments on your code, such as

# If animal is not present, set mean counts per sigting to zero
if totalrecs == 0:
    meancount = 0
else:
    meancount = np.mean(tab['Count'][isfocus])

Code comments such as these should generally be restricted to one line, although two or three lines would be OK for cases that require more explanation. Most importantly, comments should describe what the code is intended to do and why, not simply repeat literally what the code does. The above comment, for example, explains the purpose of the subsequent lines. In contrast, the comment below is basically useless, as it simply repeats what anyone reading the code could have already told you.

# Set x to zero
x = 0

A better option would be

# Initialize running count of individuals
x = 0

There's an art to determining how many comments are too many. I tend to personally be quite verbose with my comments, as I tend to use comments as markers to help me find the sections of my code that perform particular conceptual steps. Most of my code thus has a comment every three to five lines or so, although some would find this excessive.

Finally, make sure not to let your comments get out of date - when you update your code, you must update the corresponding comments. An out of date or incorrect comment is worse than no comment at all.

If you haven't done so already, open your mean_sightings.py file and add a few code comments.

2. Function-level definitions

All languages have conventions surrounding how to document the operations of a function. We've see lots of function definitions throughout this bootcamp whenever we looked up how to use a function like np.array (recall that you can call np.array? from an IPython notebook, for example, to see its function definition). In Python, a description of a function is known as a docstring. Many scientific Python packages use a convention similar to the below.

def get_sightings(filename, focusanimal):
    """
    Get number of sightings of a focus animal in a data set.

    Parameters
    ----------
    filename : str
        Path to file containing sightings data
    focusanimal : str
        Name of focus animal (not case sensitive)

    Returns
    -------
    result : tuple
        Tuple containing total count of number of focus animal seen and mean
        count of individuals per sighting event.

    Notes
    -----
    Data file must be csv format with Animal and Count columns containing
    animal name and count of individuals per sighting, respectively.

    """

Note a few important features of this docstring. First, it's indented, just like all of the other code within a function. Second, it starts immediately after the line defining the function, and starts and ends with three quotation marks (which, in Python, defines a multi-line string). Third, the first line of the docstring is a single line describing the high level purpose and/or function of the function. The rest of the docstring structure is not as standardized, but the above example is a basic form that's common in scientific Python packages.

As we previously discussed, a nice feature of docstrings is that they integrate nicely into the ways of getting help in IPython, for example, as well as with other documentation tools such as Sphinx that we'll mention later. See PEP257 for some additional information and standards for docstrings.

If you haven't already, add a nice docstring to your function(s) in mean_sightings.py.

3. Module-level documentation

At a higher level, you should also provide some overarching documentation of each of your Python module files. This is usually a relatively short summary, compared to a function-level docstring, that states the purposes of the module and lists what the module contains.

#!/usr/bin/env python

"""
Module containing functions to calculate mean number of sightings of a given
animal in a given sightings csv file.

Functions
---------
get_sightings - get number of sightings of focus animal in data set

"""

As with function docstrings in Python, a module docstring is set off by triple quotes and appears at the very top of the module.

If you haven't already, add a short module docstring to mean_sightings.py.

4. Package-level and user documentation

At the highest level of documentation, we find information that is intended (mostly) to be read by users of your code to gain an overview of everything that your code does. We won't talk about this level in detail, as it only matches the other three levels in importance for larger projects that are shared widely and maintained on an ongoing basis (which may not apply to many of your research projects). If you are interested in learning more about this process for Python packages, I'd suggest having a look at Sphinx, which is the tool used to create documentation for Python itself as well as most of the main scientific packages. The websites documenting core Python, matplotlib, and numpy provide some useful examples of Sphinx in use as well as some general documentation styles that you might wish to review.

Testing

OK, back to our project. We now have the file mean_sightings.py in our src directory. Now copy in your file test_mean_sightings.py, which contains the unit tests that you wrote for the functions in mean_sightings.py (or download a copy here, making sure to change the file name to start with "test").

You may recall that our test functions made use of a small data set, sightings_tab_sm.csv (which you can download here if you need to), that we created specifically for the purpose of testing our code. It can be a bit awkward deciding where to place this csv file - you could potentially put it in data, or here in the src directory, or perhaps in a subdirectory of src called tests or something like that. This is somewhat a matter of personal preference - for now, just copy and paste it here into the src directory (even though it's not technically code). You may want to create a readme file for this test data set as well so that you can remember how you created it.

Just to be sure we did everything right, go ahead and run py.test from the Terminal and make sure that your functions still pass. If they don't for some reason, you can try to debug your function or just cheat by copying the master mean_sightings.py and test files linked above into your src directory. Be sure to remove the -full portion of the file names and to add a second t to the word test in the second file name if you do this.

At this point, your project directory should look like this:

.
|-- data
|   |-- README.txt
|   |-- sightings_tab_lg.csv
|-- man
|-- results
|-- src
|   |-- mean_sightings.py
|   |-- sightings_tab_sm.csv
|   |-- test_mean_sightings.py

Add and commit these three new files (your module, test file, and test data set) to your git repository. You can commit these together, or separately if you think it would be useful to add a different commit message for the different files.

5. The runall script

Now that we have our core functions and tests in place, it's time to create the "button" for our push-button workflow - the runall.py script. The idea is that you will be able to start with an empty results directory, execute the line python runall.py in Terminal, and have our table and figure saved in the results directory.

Create a new text file called runall.py and copy and paste the following code into it.

#!/usr/bin/env python

'''
Script to create all results for camera_analysis project.
'''

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from mean_sightings import get_sightings


# ------------------------------------------------------------------------
# Declare variables
# ------------------------------------------------------------------------

# Set paths to data and results directories. Note that this method of
# relative paths only works on *nix - for Windows, see os.path module.
data_dir = '../data/'
results_dir = '../results/'

# Set name of data file, table, and figure
data_name = 'sightings_tab_lg.csv'
table_name = 'spp_table.csv'
fig_name = 'spp_fig.png'

# Set names of species to count
spp_names = ['Fox', 'Wolf', 'Grizzly', 'Wolverine']


# ------------------------------------------------------------------------
# Perform analysis
# ------------------------------------------------------------------------

# Declare empty list to hold counts of records
spp_recs = []

# Get total number of records for each species
for spp in spp_names:
    totalrecs, meancount = get_sightings(data_dir + data_name, spp)
    spp_recs.append(totalrecs)

print spp_names
print spp_recs

We won't go over this code in too much detail, as you should now have the background to understand what's happening on your own. Right up front, after importing the necessary modules, we have set up variables that define the locations of the data and results directories (relative to the src directory where our runall.py file is located) as well as the names of our input data file and the table and figure that we will create. We also declared the list of species names here. The purpose of declaring these variables up front, rather than just typing these names into the code later on where they appear, is to make it easy to change these later on if, for example, we want to analyze a different set of species or use a different data file.

After those declarations, we simply set up a for loop that goes through each of our species names and uses our previously-written function to get the number of records for that species. At the very end, we print out the lists of species names and records just to have a look at the output.

Now, go back to your Terminal window, navigate to the src directory, and run the command python runall.py. You should see the species names and records printed in your Terminal window - this shows that our code is running without errors to this point.

Now let's add some code to save the table. Erase the print lines from the bottom of runall.py, and add the text below.

# ------------------------------------------------------------------------
# Save results as table
# ------------------------------------------------------------------------

# Put two lists into a pandas DataFrame
table = pd.DataFrame(np.array(zip(spp_names, spp_recs),
                 dtype=[('species', 'S12'), ('recs', int)]))

# Save DataFrame as csv
table.to_csv(results_dir + table_name, index=False)

This code simply takes our two lists, the list of species names and of records, and creates a record array from them. The syntax to do this might seem confusing, and at this point, it's probably just best to start by memorizing it as the "recipe" that one uses to turn several lists into a record array. The dtype variable is used to name each "column" in our recarray and to tell Python the format of each column. The first column, called 'species', is given the format 'S12', which means a string of up to 12 characters. The second column, recs, is given the format int, which stands for an integer. This structured array can easily be converted into a pandas DataFrame. We then use the to_csv function to save our pandas DataFrame as a csv file. The command index=False tells pandas not to output the index column.

Once again, go back to your Terminal window and execute this file. Check to see that the csv file was correctly created and saved in the results directory.

Last but not least, let's add some code to make and save a bar chart that visually displays the data that's in our table. Add the code below to the runall file.

# -----------------------------------------------------------------------
# Save results as figure
# -----------------------------------------------------------------------

# Set up figure with one axis
fig, ax = plt.subplots(1, 1)

# Create bar chart: args are location of left edge, height, and width of bar
ax.bar([0,1,2,3], spp_recs, 0.8)

# Place tick marks in center of each bar
ax.set_xticks([0.4, 1.4, 2.4, 3.4])

# Set limits to give some white space on either side of first/last bar
ax.set_xlim([-0.2, 4])

# Add species names to x axis
ax.set_xticklabels(spp_names)

# Save figure
fig.savefig(results_dir + fig_name)

This code does just what the comments say that it does. The resulting figure looks OK, and you would probably want to spend more time adding additional lines here to adjust the formatting.

Don't forget to add runall.py to your git repo.

6. Run the push button analysis

Now with everything in place, we're ready for the magic. Just for good measure, delete any files that are hanging around in your results directory. Then, execute python runall.py from your src subdirectory and marvel at your fully reproducible workflow! (If you've been making a lot of changes to your code, and aren't quite sure what's in your results directory, you may want to periodically clear out this folder and re-run everything to make sure that everything is regenerating properly.)

At this point, your directory should look like the below.

.
|-- data
|   |-- README.txt
|   |-- sightings_tab_lg.csv
|-- man
|-- results
|   |-- spp_table.csv
|   |-- spp_fig.png
|-- src
|   |-- mean_sightings.py
|   |-- runall.py
|   |-- sightings_tab_sm.csv
|   |-- test_mean_sightings.py

At this point, a natural question to ask is whether you need to add the contents of your results directory to your git repository. The answer should be obvious - you do not need to do this, since the files in your results directory contain no unique information on their own. Everything you need to create them is contained in the data and src directories. One exception to this, though, might be if your analysis takes a very long time to run and the outputs are fairly small in size, in which case you may want to periodically commit (so that you can easily recover) the results associated with "intermediate" versions of your code.

While many of your projects will be nearly this simple, some will be more complex, sometimes significantly so. You will eventually come across the need to deal with modules that are shared across multiple projects, running the same analysis on multiple sets of parameters simultaneously, running analyses on multiple computers, etc. While we don't have time to go into these extra bits in detail, feel free to ask the instructors about any specific issues that you expect to encounter in the near future. Rest assuered that no matter how complicated your situation is, Python will provide you with a (relatively) efficient and robust way to accomplish your goals.

And that just about does it. Good luck!

Facebook Google+ Twitter GitHub RSS License Bug Report