Why Click?

There are so many libraries out there for writing command line utilities, why does click exist?

This question is easy to answer: because there is not a single command line utility for Python out there which ticks the following boxes:

• is lazily composable without restrictions
• fully follows the UNIX command line conventions
• supports loading values from environment variables out of the box
• supports for prompting of custom values
• is fully nestable and composable
• works the same on Python 2 and 3
• supports file handling out of the box

There are many alternatives to click and you can have a look at them if you enjoy them better. The obvious ones are optparse and argparse from the standard library.

click is actually implemented as a wrapper around optparse and does not implement any parsing itself. The reason it’s not based on argparse is that argparse does not allow proper nesting of commands by design and has some deficiencies when it comes to POSIX compliant argument handling.

In case optparse ever gets deprecated and removed from Python, click will ship a version of it.

Click is designed to be fun to work with and at the same time not stand in your way. It’s not overly flexible at the same time either. Currently for instance it does not allow you to customize the help pages too much. This is intentional because click is designed to allow you to nest command line utilities. The idea is that you can have a system that works together with another system by tacking two click instances together and they will continue working as they should.

Too much customizability would break this promise.

Click was written to support the Flask microframework ecosystem because no tool could provide it with the functionality it needed.

To get an understanding of what click is all about I strongly recommend looking at the Complex Applications chapter to see what it’s useful for.

Why not Argparse?

Click is internally based on optparse instead of argparse. This however is an implementation detail that a user does not have to concern himself with. The reason however click is not using argparse is that it has some problematic behaviors that make handling arbitrary command line interfaces hard:

• argparse has builtin magic behavior to guess if something is an argument or an option. This becomes a problem when dealing with incomplete command lines as it’s not possible to know without having a full understanding of the command line how the parser is going to behave. This goes against click’s ambitions of dispatching to subparsers.
• argparse currently does not support disabling of interspearsed arguments. Without this feature it’s not possible to safely implement click’s nested parsing nature.

Why not Docopt etc.?

Docopt and many tools like it are cool in how they work but very few of these tools deal with nesting of commands. To the best of the developer’s knowledge, click is the first Python library that tries to aim for composability of applications that goes beyond what the system itself supports.

For instance a Click subcommand could accept all the arguments unparsed and then be parsed with docopt if that command would want to work that way.

Why Hardcoded Behaviors?

The other question is why click goes away from optparse and hardcodes certain behaviors instead of staying configurable. There are multiple reasons for this. The biggest one is that too much configurability makes it hard to achieve a consistent command line experience.

The best example for this is optparse’s callback functionality for accepting arbitrary number of arguments. Due to syntactical ambuiguities on the command line there is no way to implement fully variadic arguments. There are always tradeoffs that need to be made and in case of argparse these tradeoffs have been critical enough, that a system like click cannot even be implemented on top of it.

In this particular case click attempts to stay with a handful of accepted paradigms for building command line interfaces that can be well documented and tested.