Hiya, Folks! Sharing what I have just learnt about the new pip's solver. pip install PROJECT no longer guarantees to install the latest version of PROJECT (or, well, giving you the ERROR that it cannot do it because something something :-) ). In fact, it will install the latest version *matching other constraints* and do it *silently*. Like it was recently only with Python version (i.e. py3-only would not get installed on py2 - that is cool) but now it moved into any-package territory. As an example, I can give you [1] where we are experimenting with getting some extracurricular package into our containers, notably fluent-logger. The only dep of fluent-logger is msgpack but the latest msgpack (as in upper constraints: 1.0.2, or any 1.x for that matter) is not compatible. However, the pin was introduced in fluent-logger in its 0.9.5 release (0.9.6 is the latest). Guess what pip does? Here is what it does: INFO:kolla.common.utils.openstack-base:Collecting fluent-logger INFO:kolla.common.utils.openstack-base: Downloading http://mirror-int.dfw.rax.opendev.org:8080/pypifiles/packages/1a/f5/e6c30ec7a81e9c32c652c684004334187db4cc09eccf78ae7b69e62c7b10/fluent_logger-0.9.6-py2.py3-none-any.whl (12 kB) INFO:kolla.common.utils.openstack-base: Downloading http://mirror-int.dfw.rax.opendev.org:8080/pypifiles/packages/d5/cb/19d838561ec210321aea24c496ec61930d6fdbb2f98d3f06cebab33c1331/fluent_logger-0.9.5-py2.py3-none-any.whl (12 kB) INFO:kolla.common.utils.openstack-base: Downloading http://mirror-int.dfw.rax.opendev.org:8080/pypifiles/packages/d1/d4/f9b3493f974cdac831bf707c3d9fec93b1a0ebd986eae4db4f101dd72378/fluent_logger-0.9.4-py2.py3-none-any.whl (12 kB) And that's it. Pip is happy, you got your "latest" version. In previous pip one would get the latest version AND a warning. Now just pip's view on what the "latest" version is. I am glad we have upper-constraints which save the day here (forcing the ERROR) but beware of this "in the wild". [1] https://review.opendev.org/c/openstack/kolla/+/759855 -yoctozepto