Writing Workflows

In Snakemake, workflows are specified as Snakefiles. Inspired by GNU Make, a Snakefile contains rules that denote how to create output files from input files. Dependencies between rules are handled implicitly, by matching filenames of input files against output files. Thereby wildcards can be used to write general rules.


The Snakefile syntax obeys the following grammar, given in extended Backus-Naur form (EBNF)

snakemake  = statement | rule | include | workdir
rule       = "rule" (identifier | "") ":" ruleparams
include    = "include:" stringliteral
workdir    = "workdir:" stringliteral
ni         = NEWLINE INDENT
ruleparams = [ni input] [ni output] [ni params] [ni message] [ni threads] [ni (run | shell)] NEWLINE snakemake
input      = "input" ":" parameter_list
output     = "output" ":" parameter_list
params     = "params" ":" parameter_list
log        = "log" ":" parameter_list
benchmark  = "benchmark" ":" statement
cache      = "cache" ":" bool
message    = "message" ":" stringliteral
threads    = "threads" ":" integer
resources  = "resources" ":" parameter_list
version    = "version" ":" statement
run        = "run" ":" ni statement
shell      = "shell" ":" stringliteral

while all not defined non-terminals map to their Python equivalents.

Depend on a Minimum Snakemake Version

From Snakemake 3.2 on, if your workflow depends on a minimum Snakemake version, you can easily ensure that at least this version is installed via

from snakemake.utils import min_version


given that your minimum required version of Snakemake is 3.2. The statement will raise a WorkflowError (and therefore abort the workflow execution) if the version is not met.

Best practices

Snakemake (>=5.11) comes with a code quality checker (a so called linter), that analyzes your workflow and highlights issues that should be solved in order to follow best practices, achieve maximum readability, and reproducibility. The linter can be invoked with

snakemake --lint

given that a Snakefile or workflow/Snakefile is accessible from your working directory. It is highly recommended to run the linter before publishing any workflow, asking questions on Stack Overflow or filing issues on Github.