This lesson is in the early stages of development (Alpha version)

Snakefiles are Python code


Teaching: 30 min
Exercises: 15 min
  • How can I automatically manage dependencies and outputs?

  • How can I use Python code to add features to my pipeline?

  • Use variables, functions, and imports in a Snakefile.

  • Learn to use the run action to execute Python code as an action.

Despite our efforts, our pipeline still has repeated content, for instance the names of output files/dependencies. Our zipf_test rule, for instance, is extremely clunky. What happens if we want to analyse books/sierra.txt as well? We’d have to update everything!

rule zipf_test:
    input:  '', 'abyss.dat', 'last.dat', 'isles.dat'
    output: 'results.txt'
    shell:  'python {input[0]} {input[1]} {input[2]} {input[3]} > {output}'

First, let’s cut down on a little bit of the clunkiness of the shell directive. One thing you’ve probably noticed is that all of our rules are using Python strings. Other data structures work too — let’s try a list:

rule zipf_test:
        books=['abyss.dat', 'last.dat', 'isles.dat']
    output: 'results.txt'
    shell:  'python {input.zipf} {input.books} > {output}'

(snakemake clean and snakemake -p should show that the pipeline still works!)

This illustrates a key feature of Snakemake. Snakefiles are just Python code. We can make our list into a variable to demonstrate this. Let’s create the variable DATS and use it in our zipf_test and dats rules.

DATS=['abyss.dat', 'last.dat', 'isles.dat']

# generate summary table
rule zipf_test:
    output: 'results.txt'
    shell:  'python {input.zipf} {input.books} > {output}'

rule dats:
    input: DATS

Try re-creating both the dats and results.txt targets (run snakemake clean in between).

When are Snakefiles executed?

The last example illustrated that we can use arbitrary Python code in our Snakefile. It’s important to understand when this code gets executed. Let’s add a print instruction to the top of our Snakefile.

print('Snakefile is being executed!')

DATS=['abyss.dat', 'last.dat', 'isles.dat']

# generate summary table
rule zipf_test:
# more output below

Now let’s clean up our workspace with snakemake clean

snakemake clean
Snakefile is being executed!
Provided cores: 1
Rules claiming more threads will be scaled down.
Job counts:
    count	jobs
    1	clean

rule clean:
    jobid: 0

Finished job 0.
1 of 1 steps (100%) done

Now let’s re-run the pipeline…

$ snakemake
Snakefile is being executed!
Provided cores: 1
Rules claiming more threads will be scaled down.
Job counts:
    count	jobs
    3	count_words
    1	zipf_test

rule count_words:
    input:, books/last.txt
    output: last.dat
    jobid: 3
    wildcards: file=last

Finished job 3.
1 of 4 steps (25%) done

rule count_words:
    input:, books/abyss.txt
    output: abyss.dat
    jobid: 1
    wildcards: file=abyss

Finished job 1.
2 of 4 steps (50%) done

rule count_words:
    input:, books/isles.txt
    output: isles.dat
    jobid: 2
    wildcards: file=isles

Finished job 2.
3 of 4 steps (75%) done

rule zipf_test:
    input:, abyss.dat, last.dat, isles.dat
    output: results.txt
    jobid: 0

Finished job 0.
4 of 4 steps (100%) done

Let’s do a dry-run:

$ snakemake -n
Snakefile is being executed!
Nothing to be done.

In every case, the print() statement ran before any of the actual pipeline code was run. What we can take away from this is that Snakemake executes the entire Snakefile every time we run snakemake (regardless of if it’s a dry run!). Because of this, we need to be careful, and only put tasks that do “real work” (changing files on disk) inside rules.

Using functions in Snakefiles

In our example here, we only have 4 books. But what if we had 700 books to be processed? It would be a massive effort to update our DATS variable to add the name of every single book’s corresponding .dat filename.

Fortunately, Snakemake ships with several functions that make working with large numbers of files much easier. The two most helpful ones are glob_wildcards() and expand(). Let’s start an interactive Python session to see how they work:

$ python3
Python 3.6.1 (default, Jun 27 2017, 14:35:15)
Type "copyright", "credits" or "license" for more information.

In this example, we will import these Snakemake functions directly in our interactive Python session. It is not necessary to import these functions within your Snakefile — these functions are always imported for you.

from import expand, glob_wildcards

Generating file names with expand()

The first function we’ll use is expand(). expand() is used quite literally, to expand a snakemake wildcard(s) into a set of filenames.

>>> expand('folder/{wildcard1}_{wildcard2}.txt',
...        wildcard1=['a', 'b', 'c'],
...        wildcard2=[1, 2, 3])

In this case, expand() created every possible combination of filenames from the two wildcards. Useful! Of course, this still leaves us needing somehow get the values for wildcard1 and wildcard2 in the first place.

Get wildcard values with glob_wildcards()

To get a set of wildcards from a list of files, we can use the glob_wildcards() function. Let’s try grabbing all of the book titles in our books folder.

>>> glob_wildcards('books/{example}.txt')
Wildcards(example=['isles', 'last', 'abyss', 'sierra'])

glob_wildcards() returns a Wildcards object as output. Wildcards is a special object defined by Snakemake that provides named lists.

In this case, there is only one wildcard, {example}. We can extract the values for the file names by getting the example property from the output of glob_wildcards()

>>> glob_wildcards('books/{example}.txt').example
['isles', 'last', 'abyss', 'sierra']

Putting it all together

Using the expand() and glob_wildcards() functions, modify the pipeline so that it automatically detects and analyses all the files in the books/ folder.

Using Python code as actions

One very useful feature of Snakemake is the ability to execute Python code instead of just shell commands. Instead of shell: as an action, we can use run: instead.

Add the following to our snakefile:

# at the top of the file
import os
import glob

# add this wherever
rule print_book_names:
        print('These are all the book names:')
        for book in glob.glob('books/*.txt'):

Upon execution of the corresponding rule, Snakemake dutifully runs our Python code in the run: block:

$ snakemake print_book_names
Provided cores: 1
Rules claiming more threads will be scaled down.
Job counts:
    count	jobs
    1	print_book_names

rule print_book_names:
    jobid: 0

These are all the book names:
Finished job 0.
1 of 1 steps (100%) done

Moving output locations

Alter the rules in your Snakefile so that the .dat files are created in their own dats/ folder. Note that creating this folder beforehand is unnecessary. Snakemake automatically creates any folders for you, as needed.

Creating PNGs

Add new rules and update existing rules to:

  • Create .png files from .dat files using
  • Remove all auto-generated files (.dat, .png, results.txt).

Finally, many Snakefiles define a default target called all as first target, that will build what the Snakefile has been written to build (e.g. in our case, the .png files and the results.txt file). Add an all target to your Snakefile (Hint: this rule has the results.txt file and the .png files as dependencies, but no actions). With that in place, instead of running snakemake results.txt, you should now run snakemake all, or just simply snakemake.

Creating an Archive

Update your pipeline to:

  • Create an archive, zipf_analysis.tar.gz, to hold all our .dat files, plots, and the Zipf summary table.
  • Update all to expect zipf_analysis.tar.gz as input.
  • Remove zipf_analysis.tar.gz when snakemake clean is called.

The syntax to create an archive is shown below:

tar -czvf zipf_analysis.tar.gz file1 directory2 file3 etc

After these exercises our final workflow should look something like the following:

Final directed acyclic graph

Adding more books

We can now do a better job at testing Zipf’s rule by adding more books. The books we have used come from the Project Gutenberg website. Project Gutenberg offers thousands of free e-books to download.

Exercise instructions

  • Go to Project Gutenberg and use the search box to find another book, for example ‘The Picture of Dorian Gray’ by Oscar Wilde.
  • Download the ‘Plain Text UTF-8’ version and save it to the books folder; choose a short name for the file
  • Optionally, open the file in a text editor and remove extraneous text at the beginning and end (look for the phrase End of Project Gutenberg's [title], by [author])
  • Run snakemake and check that the correct commands are run
  • Check the results.txt file to see how this book compares to the others

Key Points

  • Snakefiles are Python code.

  • The entire Snakefile is executed whenever you run snakemake.

  • All actual work should be done by rules.