pip only respects 1 version specifier for any particular name

[James Polley]

... and a top-level name in requirements.txt will be preferred over one inside install_requires.

The practical implication of this is that if requirements.txt specifies "bar>=2.0" and "foo", and "foo" install_requires "bar>=2.1", "pip install -r requirements.txt" will not upgrade an already-installed "bar==2.0", while "pip install foo" will.

attachment repro.tar.gz has a small virtualenv skeleton that can be used to demonstrate this.

Create and activate a virtualenv, unpack repro.tar.gz inside; you should have a requirements.txt and directory called index/ containing two versions of bar and one version of foo.

First, install bar==2.0:

    pip install -f index/ bar==2.0

Now install from requirements.txt:

    pip install -f index/ -r requirements

You should see that foo is upgraded, but bar is not:

    Successfully installed foo-0.3

Reset the environment: pip uninstall foo

Now install the package directly:

    pip install -f index/ foo

You should see that foo is installed and bar upgraded:

    Successfully installed bar-2.1 foo-0.3

[Robert Collins (lifeless)]

Ok, so this is just pip issue 988 I believe.

First encountered requirement is honoured, rest error or are ignored.

[James Polley (tchaypo)]

Yes, I believe that what I've demonstrated is pip issue 988 - specifically case 1(b) from that bug (top-level requirements override dependencies)

The reason it's filed here is because of a belief that pbr causes pip to do something different.

[Robert Collins (lifeless)]

I don't see how its different. You've supplied a top level constraint as in - 1b

lets work through this:

>First, install bar==2.0:
> pip install -f index/ bar==2.0

trivial

>Now install from requirements.txt:
> pip install -f index/ -r requirements

This is precisely equivalent to pip install -f index foo bar>=2.0
Stage 0 reqs are foo, bar>=2.0
when foo is examined 0.3 is found, giving us a selection of foo=0.3. foo's dependency of bar >=2.1 is ignored, because 1b - bar>=2.0 is already the constraint and the new constraint is ignored.

> You should see that foo is upgraded, but bar is not:
> Successfully installed foo-0.3

trivial

> Reset the environment: pip uninstall foo
> Now install the package directly:
> pip install -f index/ foo

The top level constraint here is foo, bar is not a constraint.
Now the bar constraint from foo is added, and evaluated against the environment, resulting in a mandatory upgrade from 2.0.

[Robert Collins (lifeless)]

closing, redundant with pip 988.