Different channels can have the same package, so conda must handle these channel collisions.
There will be no channel collisions if you use only the defaults channel. There will also be no channel collisions if all of the channels you use only contain packages that do not exist in any of the other channels in your list. The way conda resolves these collisions matters only when you have multiple channels in your channel list that host the same package.
Before conda 4.1.0¶
Before conda 4.1.0 was released on June 14, 2016, when two channels hosted packages with the same name, conda installed the package with the highest version number. If there were two packages with the same version number, conda installed the one with the highest build number. Only if both the version numbers and build numbers were identical did the channel ordering make a difference. This approach had 3 problems:
Build numbers from different channels are not comparable. Channel A could do nightly builds while Channel B does weekly builds, so build 2 from Channel B could be newer than build 4 from Channel A.
Users could not specify a preferred channel. You might consider Channel B more reliable than Channel A and prefer to get packages from that channel even if the B version is older than the package in Channel A. Conda provided no way to choose that behavior. Only version and build numbers mattered.
Build numbers conflicted. This is an effect of the other 2 problems. Assume you were happily using package Alpha from Channel A and package Bravo from Channel B. The provider from Channel B then added a version of Alpha with a very high build number. Your conda updates would start installing new versions of Alpha from Channel B whether you wanted that or not. This could cause unintentional problems and a risk of deliberate attacks.
After conda 4.1.0¶
By default, conda now prefers packages from a higher priority channel over any version from a lower priority channel. Therefore, you can now safely put channels at the bottom of your channel list to provide additional packages that are not in the default channels, and still be confident that these channels will not override the core package set.
Conda collects all of the packages with the same name across all listed channels and processes them as follows:
Sorts packages from highest to lowest channel priority.
Sorts tied packages---same channel priority---from highest to lowest version number.
Sorts still-tied packages---same channel priority and same version---from highest to lowest build number.
Installs the first package on the sorted list that satisfies the installation specifications.
To make conda use the old method and install the newest version of a package in any listed channel:
channel_priority: falseto your
Run the equivalent command:
conda config --set channel_priority false
Conda then sorts as follows:
Sorts the package list from highest to lowest version number.
Sorts tied packages from highest to lowest channel priority.
Sorts tied packages from highest to lowest build number.
Because build numbers from different channels are not comparable, build number still comes after channel priority.
The following command adds the channel "new_channel" to the top of the channel list, making it the highest priority:
conda config --add channels new_channel
Conda now has an equivalent command:
conda config --prepend channels new_channel
Conda also now has a command that adds the new channel to the bottom of the channel list, making it the lowest priority:
conda config --append channels new_channel