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docs: Emphasize that pip may break existing packages 

Related to #7744 .
This commit is contained in:
Sumana Harihareswara 2020-10-28 23:21:11 -04:00
parent fbcdeca91a
commit 3f4e15aef0
1 changed files with 14 additions and 15 deletions
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docs/html/user_guide.rst
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@ -197,18 +197,16 @@ In practice, there are 4 common uses of Requirements files:
py -m pip freeze > requirements.txt
py -m pip install -r requirements.txt
2. Requirements files are used to force pip to properly resolve
dependencies. In versions of pip prior to 20.3, pip `doesn't have
true dependency resolution
<https://github.com/pypa/pip/issues/988>`_, but instead simply uses
the first specification it finds for a project. E.g. if ``pkg1``
requires ``pkg3>=1.0`` and ``pkg2`` requires ``pkg3>=1.0,<=2.0``,
and if ``pkg1`` is resolved first, pip will only use ``pkg3>=1.0``,
and could easily end up installing a version of ``pkg3`` that
conflicts with the needs of ``pkg2``. To solve this problem for
pip 20.2 and previous, you can place ``pkg3>=1.0,<=2.0`` (i.e. the
correct specification) into your requirements file directly along
with the other top level requirements. Like so::
2. Requirements files are used to force pip to properly resolve dependencies.
pip 20.2 and earlier `doesn't have true dependency resolution
<https://github.com/pypa/pip/issues/988>`_, but instead simply uses the first
specification it finds for a project. E.g. if ``pkg1`` requires
``pkg3>=1.0`` and ``pkg2`` requires ``pkg3>=1.0,<=2.0``, and if ``pkg1`` is
resolved first, pip will only use ``pkg3>=1.0``, and could easily end up
installing a version of ``pkg3`` that conflicts with the needs of ``pkg2``.
To solve this problem, you can place ``pkg3>=1.0,<=2.0`` (i.e. the correct
specification) into your requirements file directly along with the other top
level requirements. Like so::
pkg1
pkg2
@ -1441,12 +1439,13 @@ time to fix the underlying problem in the packages, because pip will
be stricter from here on out.
This also means that, when you run a ``pip install`` command, pip only
considers the packages you are installing in that command, and may
break already-installed packages. It will not guarantee that your
considers the packages you are installing in that command, and **may
break already-installed packages**. It will not guarantee that your
environment will be consistent all the time. If you ``pip install x``
and then ``pip install y``, it's possible that the version of ``y``
you get will be different than it would be if you had run ``pip
install x y`` in a single command. We would like your thoughts on what
install x y`` in a single command. We are considering changing this
behavior (per :issue:`7744`) and would like your thoughts on what
pip's behavior should be; please answer `our survey on upgrades that
create conflicts`_.