qer 0.8.0

Python requirements compiler

Qer Python Requirements Compiler

Qer is a Python work-in-progress requirements compiler geared toward large Python projects. It allows you to:

• Produce an output file consisting of fully constrained exact versions of your requirements

• Identify sources of constraints on your requirements

• Constrain your output requirements using requirements that will not be included in the output

• Save distributions that are downloaded while compiling

• Use a current solution as a source of requirements. In other words, you can easily compile a subset from an existing solution.

Why use it?

pip-tools is the defacto requirements compiler for Python, but is missing some important features.

• Does not allow you to use constraints that are not included in the final output

• Provides no tools to track down where conflicting constraints originate

• Cannot treat source directories recursively as package sources

Qer has these features, making it an effective tool for large Python projects.

This situation is very common:

You have a project with requirements requirements.txt and test requirements test-requirements.txt. You want to produce a fully constrained output of requirements.txt to use to deploy your application. Easy, right? Just compile requirements.txt. However, if your test requirements will in any way constrain packages you need, even those needed transitively, it means you will have tested with different versions than you’ll ship.

For this reason, you can user Qer to compile requirements.txt using test-requirements.txt as constraints.

The Basics

Install and run

Qer can be simply installed by running:

pip install qer

Two entrypoint scripts are provided:

req-compile <input reqfile1> ... <input_reqfileN> [--constraints constraint_file] [--index-url https://...]
req-hash <input reqfile1> ... <input_reqfileN>

Producing output requirements

To produce a fully constrained set of requirements for a given number of input requirements files, pass requirements files to req-compile:

> cat requirements.txt
astroid>=2.0.0
isort >= 4.2.5
mccabe

> req-compile requirements.txt
astroid==2.1.0                          #
futures==3.2.0                          # isort
isort==4.3.4                            #
lazy-object-proxy==1.3.1                #
mccabe==0.6.1                           #
six==1.12.0                             # astroid
typing==3.6.6                           # astroid
wrapt==1.11.1                           # astroid

Output is always emitted to stdout. Possible inputs include:

> req-compile
> req-compile .
# Compiles the current directory (looks for a setup.py)

> req-compile .[test]
# Compiles the current directory with the extra "test"

> req-compile subdir/project
# Compiles the project in the subdir/project directory

> req-compile subdir/project2[test,docs]
# Compiles the project in the subdir/project2 directory with the test and docs extra requirements included

> req-candidates --paths-only | req-compile
# Search for candidates and compile them piped in via stdin

> echo flask | req-compile
# Compile the requirement 'flask' using the defaut remote index (PyPI)

Specifying source of distributions

Qer supports obtaining python distributions from multiple sources, each of which can be specified more than once. The following sources can be specified, resolved in the same order (e.g. source takes precedence over index-url):

• --solution

  Load a previous solution and use it as a source of distributions. This will allow a full recompilation of a working solution without requiring any other source. If the solution file can’t be found, a warning will be emitted but not cause a failure

• --source

  Use a local filesystem with source python packages to compile from. This will search the entire tree specified at the source directory, until an __init__.py is reached. --remove-source can be supplied to remove results that were obtained from source directories. You may want to do this if compiling for a project and only third party requirements compilation results need to be saved.

• --find-links

  Read a directory to load distributions from. The directory can contain anything a remote index would, wheels, zips, and source tarballs. This matches pip’s commmand line.

• --index-url

  URL of a remote index to search for packages in. When compiling, it’s necessary to download a package to determine its requirements. --wheel-dir can be supplied to specify where to save these distributions. Otherwise they will be deleted after compilation is complete.

All options can be repeated multiple times, with the resolution order within types matching what was passed on the commandline. However, overall resolution order will always match the order of the list above.

By default, PyPI (https://pypi.org/) is added as a default source. It can be removed by passing --no-index on the commandline.

Identifying source of constraints

Why did I just get version 1.11.0 of six? Find out by examining the output:

six==1.11.0  # astroid, pathlib2, pymodbus (==1.11.0), pytest (>=1.10.0), more_itertools (<2.0.0,>=1.0.0)

Hashing input requirements

Hash input requirements by allowing Qer to parse, combine, and hash a single list. This will allow multiple input files to be logically combined so irrelevant changes don’t cause recompilations. For example, adding tenacity to a nested requirements file when tenacity is already included elsewhere.:

> req-hash projectreqs.txt
dc2f25c1b28226b25961a5320e25c339e630342d0ce700b126a5857eeeb9ba12

Constraining output

Constrain production outputs with test requirements using the --constraints flag. More than one file can be passed:

> cat requirements.txt
astroid

> cat test-requirements.txt
pylint<1.6

> req-compile requirements.txt --constraints test-requirements.txt
astroid==1.4.9                          # (via constraints: pylint (<1.5.0,>=1.4.5))
lazy-object-proxy==1.3.1                # astroid
six==1.12.0                             # astroid
wrapt==1.11.1                           # astroid

Note that astroid is constrained by pylint, even though pylint is not included in the output.

Advanced Features

Compiling a constrained subset

Input can be supplied via stdin as well as via as through files. For example, to supply a full solution through a second compilation in order to obtain a subset of requirements, the following cmdline might be used:

> req-compile requirements.txt --constraints compiled-requirements.txt

or, for example to consider two projects together:

> req-compile /some/other/project /myproject | req-compile /myproject --solution -

which is equivalent to:

> req-compile /myproject --constraints /some/other/project

Resolving constraint conflicts

Conflicts will automatically print the source of each conflicting requirement:

> cat projectreqs.txt
astroid<1.6
pylint>=1.5

> req-compile projectreqs.txt
No version of astroid could satisfy the following requirements:
   projectreqs.txt requires astroid<1.6
   pylint 1.9.4 (via projectreqs.txt (>=1.5)) requires astroid<2.0,>=1.6

Saving distributions

Files downloading during the compile process can be saved for later install. This can optimize the execution times of builds when a separate compile step is required:

> req-compile projectreqs.txt --wheel-dir .wheeldir > compiledreqs.txt
> pip install -r compilereqs.txt --find-links .wheeldir --no-index

Cookbook

Some useful patterns for projects are outlined below.

Compile, then install

After requirements are compiled, the usual next step is to install them into a virtualenv.

A script for test might run:

> req-compile --extra test --solution compiled-requirements.txt --wheel-dir .wheeldir > compiled-requirements.txt
> pip-sync compiled-requirement.txt --find-links .wheeldir --no-index
or
> pip install -r compiled-requirements.txt --find-links .wheeldir --no-index

This would produce an environment containing all of the requirements and test requirements for the project in the current directory (as defined by a setup.py). This is a stable set, in that only changes to the requirements and constraints would produce a new output. To produce a totally fresh compilation, don’t pass in a previous solution.

The find-links parameter to the sync or pip install will reuse the wheels already downloaded by Qer during the compilation phase. This will make the installation step entirely offline.

When taking this environment to deploy, trim down the set to the install requirements:

> req-compile --solution compiled-requirements.txt --no-index > install-requirements.txt

install-requirements.txt will contain the pinned requirements that should be installed in your target environment. The reason for this extra step is that you don’t want to distribute your test requirements, and you also want your installed requirements to be the same versions that you’ve tested with. In order to get all of your explicitly declared requirements and all of the transitive dependencies, you can use the prior solution to extract a subset. Passing the --no-index makes it clear that this command will not hit the remote index at all (though this would naturally be the case as solution files take precedence over remote indexes in repository search order).

Compile for a group of projects

Qer can discover requirements that are grouped together on the filesystem. The req-candidates command will print discovered projects and with the --paths-only options will dump their paths to stdout. This allows recursive discovery of projects that you may want to compile together.

For example, consider a filesystem with this layout:

solution
  \_ utilities
  |   \_ network_helper
  |_ integrations
  |   \_ github
  \_ frameworks
      |_ neural_net
      \_ cluster

In each of the leaf nodes, there is a setup.py and full python project. To compile these together and ensure that their requirements will all install into the same environment:

> cd solution
> req-candidates --paths-only
/home/user/projects/solution/utilities/network_helper
/home/user/projects/solution/integrations/github
/home/user/projects/solution/frameworks/neural_net
/home/user/projects/solution/frameworks/cluster

> req-candidates --paths-only | req-compile --extra test --solution compiled-requirements.txt --wheel-dir .wheeldir > compiled-requirements.txt
.. all reqs and all test reqs compiled together...