Modern robots.txt Parser for Python
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README.md

Robots Exclusion Protocol Parser for Python

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Robots.txt parsing in Python.

Goals

  • Fetching -- helper utilities for fetching and parsing robots.txts, including checking cache-control and expires headers
  • Support for newer features -- like Crawl-Delay and Sitemaps
  • Wildcard matching -- without using regexes, no less
  • Performance -- with >100k parses per second, >1M URL checks per second once parsed
  • Caching -- utilities to help with the caching of robots.txt responses

Installation

reppy is available on pypi:

pip install reppy

When installing from source, there are submodule dependencies that must also be fetched:

git submodule update --init --recursive
make install

Usage

Checking when pages are allowed

Two classes answer questions about whether a URL is allowed: Robots and Agent:

from reppy.robots import Robots

# This utility uses `requests` to fetch the content
robots = Robots.fetch('http://example.com/robots.txt')
robots.allowed('http://example.com/some/path/', 'my-user-agent')

# Get the rules for a specific agent
agent = robots.agent('my-user-agent')
agent.allowed('http://example.com/some/path/')

The Robots class also exposes properties expired and ttl to describe how long the response should be considered valid. A reppy.ttl policy is used to determine what that should be:

from reppy.ttl import HeaderWithDefaultPolicy

# Use the `cache-control` or `expires` headers, defaulting to a 30 minutes and
# ensuring it's at least 10 minutes
policy = HeaderWithDefaultPolicy(default=1800, minimum=600)

robots = Robots.fetch('http://example.com/robots.txt', ttl_policy=policy)

Customizing fetch

The fetch method accepts *args and **kwargs that are passed on to requests.get, allowing you to customize the way the fetch is executed:

robots = Robots.fetch('http://example.com/robots.txt', headers={...})

Matching Rules and Wildcards

Both * and $ are supported for wildcard matching.

This library follows the matching 1996 RFC describes. In the case where multiple rules match a query, the longest rules wins as it is presumed to be the most specific.

Checking sitemaps

The Robots class also lists the sitemaps that are listed in a robots.txt

# This property holds a list of URL strings of all the sitemaps listed
robots.sitemaps

Delay

The Crawl-Delay directive is per agent and can be accessed through that class. If none was specified, it's None:

# What's the delay my-user-agent should use
robots.agent('my-user-agent').delay

Determining the robots.txt URL

Given a URL, there's a utility to determine the URL of the corresponding robots.txt. It preserves the scheme and hostname and the port (if it's not the default port for the scheme).

# Get robots.txt URL for http://userinfo@example.com:8080/path;params?query#fragment
# It's http://example.com:8080/robots.txt
Robots.robots_url('http://userinfo@example.com:8080/path;params?query#fragment')

Caching

There are two cache classes provided -- RobotsCache, which caches entire reppy.Robots objects, and AgentCache, which only caches the reppy.Agent relevant to a client. These caches duck-type the class that they cache for the purposes of checking if a URL is allowed:

from reppy.cache import RobotsCache
cache = RobotsCache(capacity=100)
cache.allowed('http://example.com/foo/bar', 'my-user-agent')

from reppy.cache import AgentCache
cache = AgentCache(agent='my-user-agent', capacity=100)
cache.allowed('http://example.com/foo/bar')

Like reppy.Robots.fetch, the cache constructory accepts a ttl_policy to inform the expiration of the fetched Robots objects, as well as *args and **kwargs to be passed to reppy.Robots.fetch.

Caching Failures

There's a piece of classic caching advice: "don't cache failures." However, this is not always appropriate in certain circumstances. For example, if the failure is a timeout, clients may want to cache this result so that every check doesn't take a very long time.

To this end, the cache module provides a notion of a cache policy. It determines what to do in the case of an exception. The default is to cache a form of a disallowed response for 10 minutes, but you can configure it as you see fit:

# Do not cache failures (note the `ttl=0`):
from reppy.cache.policy import ReraiseExceptionPolicy
cache = AgentCache('my-user-agent', cache_policy=ReraiseExceptionPolicy(ttl=0))

# Cache and reraise failures for 10 minutes (note the `ttl=600`):
cache = AgentCache('my-user-agent', cache_policy=ReraiseExceptionPolicy(ttl=600))

# Treat failures as being disallowed
cache = AgentCache(
    'my-user-agent',
    cache_policy=DefaultObjectPolicy(ttl=600, lambda _: Agent().disallow('/')))

Development

A Vagrantfile is provided to bootstrap a development environment:

vagrant up

Alternatively, development can be conducted using a virtualenv:

virtualenv venv
source venv/bin/activate
pip install -r requirements.txt

Tests

Tests may be run in vagrant:

make test

Development

Environment

To launch the vagrant image, we only need to vagrant up (though you may have to provide a --provider flag):

vagrant up

With a running vagrant instance, you can log in and run tests:

vagrant ssh
make test

Running Tests

Tests are run with the top-level Makefile:

make test

PRs

These are not all hard-and-fast rules, but in general PRs have the following expectations:

  • pass Travis -- or more generally, whatever CI is used for the particular project
  • be a complete unit -- whether a bug fix or feature, it should appear as a complete unit before consideration.
  • maintain code coverage -- some projects may include code coverage requirements as part of the build as well
  • maintain the established style -- this means the existing style of established projects, the established conventions of the team for a given language on new projects, and the guidelines of the community of the relevant languages and frameworks.
  • include failing tests -- in the case of bugs, failing tests demonstrating the bug should be included as one commit, followed by a commit making the test succeed. This allows us to jump to a world with a bug included, and prove that our test in fact exercises the bug.
  • be reviewed by one or more developers -- not all feedback has to be accepted, but it should all be considered.
  • avoid 'addressed PR feedback' commits -- in general, PR feedback should be rebased back into the appropriate commits that introduced the change. In cases, where this is burdensome, PR feedback commits may be used but should still describe the changed contained therein.

PR reviews consider the design, organization, and functionality of the submitted code.

Commits

Certain types of changes should be made in their own commits to improve readability. When too many different types of changes happen simultaneous to a single commit, the purpose of each change is muddled. By giving each commit a single logical purpose, it is implicitly clear why changes in that commit took place.

  • updating / upgrading dependencies -- this is especially true for invocations like bundle update or berks update.
  • introducing a new dependency -- often preceeded by a commit updating existing dependencies, this should only include the changes for the new dependency.
  • refactoring -- these commits should preserve all the existing functionality and merely update how it's done.
  • utility components to be used by a new feature -- if introducing an auxiliary class in support of a subsequent commit, add this new class (and its tests) in its own commit.
  • config changes -- when adjusting configuration in isolation
  • formatting / whitespace commits -- when adjusting code only for stylistic purposes.

New Features

Small new features (where small refers to the size and complexity of the change, not the impact) are often introduced in a single commit. Larger features or components might be built up piecewise, with each commit containing a single part of it (and its corresponding tests).

Bug Fixes

In general, bug fixes should come in two-commit pairs: a commit adding a failing test demonstrating the bug, and a commit making that failing test pass.

Tagging and Versioning

Whenever the version included in setup.py is changed (and it should be changed when appropriate using http://semver.org/), a corresponding tag should be created with the same version number (formatted v<version>).

git tag -a v0.1.0 -m 'Version 0.1.0

This release contains an initial working version of the `crawl` and `parse`
utilities.'

git push --tags origin