Setting Up Pants

Pants will work out of the box, at least for basic functionality. However some uses require further setup.

Source Roots

Pants needs to know where the root of a source tree is, so it can correctly set up import paths, bundle code etc.

In typical cases, Pants can deduce the source roots automatically based on naming conventions. E.g., src/<lang>, src/main/<lang>, test/<lang>, src/test/<lang>, 3rdparty/<lang> and so on. However if your source roots don't conform to any of the default patterns, you can add your own patterns. See ./pants help-advanced source for details.


If you use Git as your SCM you'll want to add the following entries to your .gitignore file:


BUILD files

BUILD files describe the dependency graph of your codebase. You'll need to create some in order to use Pants.

If you're converting a mvn-built workspace to Pants, you can get a head start by using information from your pom.xml files. The mvn2pants repo demonstrates some scripts that convert pom.xml files into BUILD files. This can get you up and running with Pants faster. Careful, though: Maven projects tend to be very coarse-grained. Once you have things working, you'll probably want to break down the large Maven-style targets into finer-grained ones.


Pants is very configurable and has literally hundreds of tuneable parameters, known in Pants parlance as options.

Most options are automatically set to useful defaults. However it is inevitable that sooner or later you'll need to tweak some of them.


Pants calls out to other tools for many build steps. E.g., it uses Zinc for incremental compilation of Java and Scala code.

Pants bootstraps such tools as needed. The default versions of these tools can be overridden using the appropriate options. It's idiomatic, but not required, to set these options to addresses of targets in a special BUILD file named in the repo root.

Pants uses Ivy to fetch JVM-based tools. Ivy requires an XML configuration file. By default, pants uses the configuration that ships with Ivy, but if you need to change Ivy settings you can specify your own in pants.ini:

ivy_settings: %(pants_supportdir)s/ivy/ivysettings.xml
cache_dir: ~/.ivy2/pants

For more information on Ivy settings, see the Ivy documentation

Build Cache

Pants automatically caches its work locally. But most organizations will want to set up a shared remote build cache. To do so, set cache options in pants.ini:

read_from: ['']
write_to: ['']

Note that the read and write cache locations are separate options: you may, for example, only want your CI machines, not individual developer builds, to populate the shared cache.

Valid option values are:

  • [ '' ] RESTful server URL.
  • [ '/tmp/pantscache' ] local directory.
  • [ '|' ] pipe-separated list of URLs--Pants pings each and uses the fastest.
  • [ '/tmp/pantscache', '' ] try local fs first, then remote server.

For details see the _do_create_artifact_cache docstring in

The remote server should be a RESTful server that can handle GET, HEAD, PUT, and DELETE requests on large binary blobs. E.g., you can implement this using Varnish in front of nginx (with dav_methods PUT DELETE; enabled).

When reading from a cache server, Pants tries to GET an URL at some path under the server URL; your server should respond with 404 if there's nothing there, 200 if there is.

When writing to a cache server, Pants first tries a HEAD request to see if the file's already there; respond with 404 if there's nothing there, 200 if there is. If Pants gets a 404, it will then try a PUT request; store the file it sends.

If the user's .netrc has authentication information for the cache server[s], Pants will use it. (Thus, if only some users with known-good setups should be able to write to the cache, you might find it handy to use .netrc to authenticate those users.)

Uploading Timing Stats

Pants tracks information about its performance: what it builds, how much time various build operations take, cache hits, and more. If you you work with a large engineering organization, you might want to gather this information in one place, so it can inform decisions about how to improve everybody's build times.

To upload these stats to a server for future analysis set the following option in pants.ini:

stats_upload_url: ""

Pants will POST JSON data to that URL. The JSON format should be self-explanatory.

Using Pants behind a firewall

Pants may encounter issues running behind a firewall. Several components expect to be able to reach the Internet:

  • Ivy bootstrapper
  • Binary tool bootstrapping
  • Ivy itself (used for tool bootstrapping and downloading external .jar files)
  • Python requirements

Configuring the Python requests library

Code in and other parts of Pants use the Python requests library to download resources using http or https. The first time you may encounter this is when Pants attempts to download an initial version of Ivy. If this initial download is through a proxy, the requests library uses the HTTP_PROXY or HTTPS_PROXY environment variable to find the proxy server.

export HTTP_PROXY=

If Pants tries to find resources with HTTPS urls, you may see an error like:

Exception message: Problem fetching the ivy bootstrap jar! Problem GETing data from [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:581)

The requests library attempts to verify SSL certificates by default. The reason it is denying the request is that it cannot find a trusted public key for the root certificate authority presented by the server. The requests library uses the 'certifi' library of well known certificate authorities, if that library is installed. If you are using a virtualenv installation of pants, using pip install certifi to add the certify package to your pants environment might help. You can also download a .pem bundle from the certifi project page and set REQUESTS_CA_BUNDLE as mentioned below.

If you are using hosts with a self-signed certificate, your certificate authority will not be available in the certifi library. You will need a .pem file for the local certificate authority.

You can tell the requests library about your trusted certificate authority certificates by setting the environment variable REQUESTS_CA_BUNDLE. This variable should point to a file containing trusted certificates:

export REQUESTS_CA_BUNDLE=/etc/certs/latest.pem

Bootstrapping Ivy

Pants fetches the Ivy tool with an initial manual bootstrapping step using the Python requests library. If you do not want to use HTTP_PROXY or HTTPS_PROXY as described above, you can re-redirect this initial download to another URL with a setting in pants.ini:


You may also encounter issues downloading this .jar file if you are using self-signed SSL certificates. See the section on Configuring the Python requests library above.

Nexus as proxy

If your site uses Sonatype Nexus or another reverse proxy for artifacts, you do not need to use a separate HTTP proxy. Contact the reverse proxy administrator to setup a proxy for the sites listed in build-support/ivy/settings.xml and pants.ini. By default these are and

Here is an excerpt of a modified ivysettings.xml with some possible configurations:

<macrodef name="_remote_resolvers">
    <chain returnFirst="true">
      <ibiblio name="example-corp-maven"
      <ibiblio name=""

Redirecting tool downloads to other servers

For the binary support tools like protoc, you will need to setup a proxy for the repo, or create your own repo of build tools:

pants_support_baseurls = [

Redirecting python requirements to other servers

For python repos, you need to override the following settings in pants.ini:

repos: [

indices: [

You can also reference a local repo relative to your project's build root with this pattern:

repos: [
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