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more explanation of the install order

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Marcus Smith 2015-04-01 21:21:19 -07:00
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commit 30ea101678
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docs/reference/pip_install.rst
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@ -19,9 +19,14 @@ Description
Installation Order
++++++++++++++++++
Pip installs dependencies before the things that depend on them. In the event
of a dependency cycle, the first encountered member of the cycle is installed
last.
As of v6.1.0, pip installs dependencies before their dependents, i.e. in
"topological order". This is the only commitment pip currently makes related
to order. While it may be coincidentally true that pip will install things in
the order of the install arguments or in the order of the items in a
requirements file, this is not a promise.
In the event of a dependency cycle (aka "circular dependency"), the first
encountered member of the cycle is installed last.
For instance, if quux depends on foo which depends on bar which depends on baz,
which depends on foo::
@ -35,6 +40,30 @@ which depends on foo::
Installing collected packages foo, baz, bar
Prior to v6.1.0, pip made no commitments about install order.
The decision to install topologically is based on the principle that
installations should proceed in a way that leaves the environment usable at each
step. This has two main practical benefits:
1. Concurrent use of the environment during the install is more likely to work.
2. A failed install won't leave broken packages. Although pip would like to
support failure rollbacks eventually, in the mean time, this is an
improvement.
Although the new install order is not intended to replace (and does not replace)
the use of ``setup_requires`` to declare build dependencies, it may help certain
projects install from sdist (that might previously fail) that fit the following
profile:
1. They have build dependencies that are also declared as install dependencies
using ``install_requires``.
2. ``python setup.py egg_info`` works without their build dependencies being
installed.
3. For whatever reason, they don't or won't declare their build dependencies using
``setup_requires``.
.. _`Requirements File Format`:
Requirements File Format