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Cache output of idempotent jobs.

Project description

# Jobstamps

Make-like caching of idempotent functions for python.

This module provides memoization of long-running functions which have clearly
documented side effects and do not change their result if their inputs
have not changed. It is ideal for tools which analyze text files to produce
some output, such as a source code linter. The result of a the function
is stored in a file which is named by the hash of the function's

A separate `jobstamp` command line utility is provided for integration
with shell scripts or non-python commands. This utility caches the
standard input, output and error of command line invocation and upon
running that utility with the same arguments, the cached output
is printed and return code returned.

## Status

| Travis CI (Ubuntu) | AppVeyor (Windows) | Coverage | PyPI | Licence |

## Usage

usage: jobstamp [-h] [--dependencies [PATH [PATH ...]]]
[--output-files [PATH [PATH ...]]]
[--stamp-directory DIRECTORY] [--use-hashes]

Cache results from jobs

optional arguments:
-h, --help show this help message and exit
--dependencies [PATH [PATH ...]]
A list of paths which, if more recent than the last
time this job was invoked, will cause the job to be
--output-files [PATH [PATH ...]]
A list of expected output paths form this command,
which, if they do not exist, will cause the job to
be re-invoked.
--stamp-directory DIRECTORY
A directory to store cached results from this
If a matching invocation is used and the files
specified in --dependencies and --output-files are
up-to-date, then the cached stdout, stderr and
return code is used and the command is not run
--use-hashes Use hash comparison in order to determine if
dependencies have changed since the last invocation
of the job. This method is slower, but can
withstand files being copied or moved.

## API Usage

Python modules can integrate directly with the jobstamp API, which is
exposed as so:, *args, **kwargs)

The default signature allows for the specified function to be applied to
the specified args and kwargs. The result of the function will be cached
(so long as it can be represented in text form and parsed from its
__repr__) in a stamp file in the temporary files directory. The next time
the function is invoked through the `jobstamp` wrapper with the same arguments,
the result from the stampfile will be loaded and returned directly.

If you want to check if a function will be run again without actually running
it, then, you can use the `out_of_date` function. That function returns
either `None` or any file which would, by virtue of being out of date,
cause the job to be re-run.

out_of_date(func, *args, **kwargs)

Certain `kwargs` have special meanings and will be parsed and removed
from the `kwargs` passed to the underlying function. Those are:

- `jobstamps_dependencies`: A list of files for which this function depends
on to produce its output. If any of these files
have been updated since the last invocation, the
function will be run again.
- `jobstamps_output_files`: A list of files for which this function produces
as a side-effect. If any of these files don't
exist, the job gets run again.
- `jobstamps_cache_output_directory`: Where to store internal cached
invocation stamps. Usually this
should be specified on a per-domain
basis to avoid clashes stamps in the
global temporary files directory.
- `jobstamps_method`: Either one of `jobstamp.HashMethod` or
`jobstamp.MTimeMethod`, defaulting to the latter if
left unspecified. This option allows the user to pick
the implementation of determining whether a dependency
is out of date. `jobstamp.MTimeMethod` uses the
file-system modification time to determine if a
dependency is more recent than the last run of the
function. `jobstamp.HashMethod` uses the SHA1 algorithm
to store a hash of the file and compares the hash on
the next invocation. It is slower than
`jobstamp.MTimeMethod` but handles cases where files
are copied or otherwise saved and restored between

## Influential environment variables

Specify `JOBSTAMPS_DISABLED` to always disable caching of jobs on all
invocations. Jobs will always be re-run, but existing stamp files
won't be removed.

Specify `JOBSTAMPS_DEBUG` to see when a job was re-run or a cached
value was used.

Specify `JOBSTAMPS_ALWAYS_USE_HASHES` to force any underlying jobstamp
library to use `jobstamp.HashMethod` instead of `jobstamp.MTimeMethod`, even
if the user explicitly asked for the latter. This is useful for CI environments
where the latter method almost never works the way one would expect it to.

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