About API interfaces

Semantic versioning

b2sdk follows Semantic Versioning policy, so in essence the version number is MAJOR.MINOR.PATCH (for example 1.2.3) and:

  • we increase MAJOR version when we make incompatible API changes

  • we increase MINOR version when we add functionality in a backwards-compatible manner, and

  • we increase PATCH version when we make backwards-compatible bug fixes (unless someone relies on the undocumented behavior of a fixed bug)

Therefore when setting up b2sdk as a dependency, please make sure to match the version appropriately, for example you could put this in your requirements.txt to make sure your code is compatible with the b2sdk version your user will get from pypi:


Interface versions

You might notice that the import structure provided in the documentation looks a little odd: from b2sdk.v2 import .... The .v2 part is used to keep the interface fluid without risk of breaking applications that use the old signatures. With new versions, b2sdk will provide functions with signatures matching the old ones, wrapping the new interface in place of the old one. What this means for a developer using b2sdk, is that it will just keep working. We have already deleted some legacy functions when moving from .v0 to .v1, providing equivalent wrappers to reduce the migration effort for applications using pre-1.0 versions of b2sdk to fixing imports.

It also means that b2sdk developers may change the interface in the future and will not need to maintain many branches and backport fixes to keep compatibility of for users of those old branches.

Interface version compatibility

A numbered interface will not be exactly identical throughout its lifespan, which should not be a problem for anyone, however just in case, the acceptable differences that the developer must tolerate, are listed below.


The exception hierarchy may change in a backwards compatible manner and the developer must anticipate it. For example, if b2sdk.v2.ExceptionC inherits directly from b2sdk.v2.ExceptionA, it may one day inherit from b2sdk.v2.ExceptionB, which in turn inherits from b2sdk.v2.ExceptionA. Normally this is not a problem if you use isinstance() and super() properly, but your code should not call the constructor of a parent class by directly naming it or it might skip the middle class of the hierarchy (ExceptionB in this example).


Even in the same interface version, objects/classes/enums can get additional fields and their representations such as as_dict() or __repr__ (but not __str__) may start to contain those fields.

Methods and functions can start accepting new optional arguments. New methods can be added to existing classes.


Some effort will be put into keeping the performance of the old interfaces, but in rare situations old interfaces may end up with a slightly degraded performance after a new version of the library is released. If performance target is absolutely critical to your application, you can pin your dependencies to the middle version (using b2sdk>=X.Y.0,<X.Y+1.0) as b2sdk will increment the middle version when introducing a new interface version if the wrapper for the older interfaces is likely to affect performance.

Public interface

Public interface consists of public members of modules listed in Public API section. This should be used in 99% of use cases, it’s enough to implement anything from a console tool to a FUSE filesystem.

Those modules will generally not change in a backwards-incompatible way between non-major versions. Please see interface version compatibility chapter for notes on what changes must be expected.


If the current version of b2sdk is 4.5.6 and you only use the public interface, put this in your requirements.txt to be safe:



b2sdk.*._something and b2sdk.*.*._something, while having a name beginning with an underscore, are NOT considered public interface.

Internal interface

Some rarely used features of B2 cloud are not implemented in b2sdk. Tracking usage of transactions and transferred data is a good example - if it is required, additional work would need to be put into a specialized internal interface layer to enable accounting and reporting.

b2sdk maintainers are very supportive in case someone wants to contribute an additional feature. Please consider adding it to the sdk, so that more people can use it. This way it will also receive our updates, unlike a private implementation which would not receive any updates unless you apply them manually ( but that’s a lot of work and we both know it’s not going to happen). In practice, an implementation can be either shared or will quickly become outdated. The license of b2sdk is very permissive, but when considering whether to keep your patches private or public, please take into consideration the long-term cost of keeping up with a dynamic open-source project and/or the cost of missing the updates, especially those related to performance and reliability (as those are being actively developed in parallel to documentation).

Internal interface modules are listed in API Internal section.


It is OK for you to use our internal interface (better that than copying our source files!), however, if you do, please pin your dependencies to middle version, as backwards-incompatible changes may be introduced in a non-major version.

Furthermore, it would be greatly appreciated if an issue was filed for such situations, so that b2sdk interface can be improved in a future version in order to avoid strict version pinning.


If the current version of b2sdk is 4.5.6 and you are using the internal interface, put this in your requirements.txt: