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FAQ for Kivy Framework

Introduction

Kivy is an open-source Python framework for developing GUI apps that work cross-platform, including desktop, mobile and embedded platforms.

Sibling Projects with FAQs:

As well as the Kivy framework, there are a number of sibling projects maintained by the same team. Some have their own FAQs, including:

FAQ for Buildozer: a development tool for turning Python applications into binary packages ready for installation on any of a number of platforms, including mobile devices.
FAQ for Plyer: a platform-independent Python API for accessing hardware features of various platforms (Android, iOS, macOS, Linux and Windows).
FAQ for PyJNIus: a Python library for accessing Java classes using the Java Native Interface (JNI).
FAQ for python-for-android: a development tool that packages Python apps into binaries that can run on Android devices.
FAQ for Kivy for iOS: a toolchain to compile the necessary libraries for iOS to run Kivy applications, and manage the creation of Xcode projects.

Project Questions

Why do you use Python? Isn't it slow?

Let us try to give a thorough answer; please bear with us.

Python is a very agile language that allows you to do many things in a (by comparison) short time. For many development scenarios, we strongly prefer writing our application quickly in a high-level language such as Python, testing it, then optionally optimizing it.

But what about speed? If you compare execution speeds of implementations for a certain set of algorithms (esp. number crunching) you will find that Python is a lot slower than say, C++. Now you may be even more convinced that it's not a good idea in our case to use Python. Drawing sophisticated graphics (and we are not talking about your grandmother's OpenGL here) is computationally quite expensive and given that we often want to do that for rich user experiences, that would be a fair argument. But, in virtually every case your application ends up spending most of the time (by far) executing the same part of the code. In Kivy, for example, these parts are event dispatching and graphics drawing. Now Python allows you to do something to make these parts much faster.

By using Cython, you can compile your code down to the C level, and from there your usual C compiler optimizes things. This is a pretty pain free process and if you add some hints to your code, the result becomes even faster. We are talking about a speed-up in performance by a factors like 30 or much more; it greatly depends on your code. In Kivy, we did this for you and implemented the portions of our code, where efficiency really is critical, on the C level.

For graphics drawing, we also leverage today's GPUs which are, for some tasks such as graphics rasterization, much more efficient than a CPU. Kivy does as much as is reasonable on the GPU to maximize performance. If you use our Canvas API to do the drawing, there is even a compiler that we invented which optimizes your drawing code automatically. If you keep your drawing mostly on the GPU, much of your program's execution speed is not determined by the programming language used, but by the graphics hardware you throw at it.

We believe that these (and other) optimizations that Kivy does for you already make most applications fast enough by far. Often you will even want to limit the speed of the application in order not to waste resources.

But even if this is not sufficient, you still have the option of using Cython for your own code to greatly speed it up.

Trust us when we say that we have given this very careful thought. We have performed many different benchmarks and come up with some clever optimizations to make your application run smoothly.

What versions of Python does Kivy support?

Typically, all the versions of Python that haven't reached End of Life.

There may be some delays as new versions of Python are released, while we wait for our dependencies to add support.

To find the answer for the latest released versions, check the Python Package Index (PyPI) for Kivy. If you are developing app for other platforms, check Buildozer, python-for-android, and kivy-ios, as appropriate.

Do you accept patches?

Yes, we love patches. Obviously we don't accept every patch. Your patch has to be consistent with our styleguide and, more importantly, make sense. Come talk to us on Discord before you come up with bigger changes, especially new features.

Make sure to read our contribution guidelines.

Does the Kivy project participate in Google's Summer of Code?

Potential students sometimes ask whether we participate in GSoC. The clear answer is: Indeed. :-)

If you want to participate as a student and want to maximize your chances of being accepted, start talking to us today and try fixing some smaller (or larger, if you can ;-) problems to get used to our workflow. If we know you can work well with us, that'd be a big plus.

Here's a checklist:

Make sure to read through the website and at least skim the documentation.
Look at the source code.
Read our contribution guidelines.
Pick an idea that you think is interesting from the ideas list (see link above) or come up with your own idea.
Do some research yourself. GSoC is not about us teaching you something and you getting paid for that. It is about you trying to achieve agreed upon goals by yourself with our support. The main driving force in this should be, obviously, yourself. Many students come up and ask what they should do. Well, we don't know because we know neither your interests nor your skills. Show us you're serious about it and take initiative.
Write a draft proposal about what you want to do. Include what you understand the current state is (very roughly), what you would like to improve and how, etc.
Discuss that proposal with us in a timely manner. Get feedback.
Be patient! Especially on Discord. We will try to get to you if we're available. If not, send an email and just wait. Most questions are already answered in the docs or somewhere else and can be found with some research. If your questions don't reflect that you've actually thought through what you're asking, it might not be well received.

How does Kivy relate to other projects:

KivyMD is a separate project to Kivy - it is run by a different team. It builds powerful and beautiful widgets for use with Kivy.

Despite having the same name, Kivy's python-for-android is not related to the python-for-android project from SL4A, Py4A, or android-python27. They are distinctly different projects with different goals. You may be able to use Py4A with Kivy, but no code or effort has been made to do so. The Kivy team feels that our python-for-android is the best solution for us going forward, and attempts to integrate with and support Py4A is not a good use of our time.

Our developers are professionals and are pretty savvy in their area of expertise. However, before Kivy came around there was (and still is) a project named PyMT that was led by our core developers. We learned a great deal from that project during the time that we developed it. In the more than two years of research and development we found many interesting ways to improve the design of our framework. We have performed numerous benchmarks and as it turns out, to achieve the great speed and flexibility that Kivy has, we had to rewrite quite a big portion of the codebase, making this a backwards-incompatible but future-proof decision. Most notable are the performance increases, which are just incredible. Kivy starts and operates just so much faster, due to these heavy optimizations.

We also had the opportunity to work with businesses and associations using PyMT. We were able to test our product on a large diversity of setups and made PyMT work on all of them. Writing a system such as Kivy or PyMT is one thing. Making it work under all these different conditions is another. We have a good background here, and brought our knowledge to Kivy.

Furthermore, since some of our core developers decided to drop their full-time jobs and turn to this project completely, it was decided that a more professional foundation had to be laid. Kivy is that foundation. It is supposed to be a stable and professional product. Technically, Kivy is not really a successor to PyMT because there is no easy migration path between them. However, the goal is the same: Producing high-quality applications for novel user interfaces. This is why we encourage everyone to base new projects on Kivy instead of PyMT.

Active development of PyMT has stalled. Maintenance patches are still accepted.

There is a defunct project in PyPI called kivy3. It was developed to enhance Kivy to add 3D graphics, but it was not developed by the Kivy team; it does not represent the successor to Kivy.

There are projects in PyPI called kivy4 and kivy5. They claim to be MIT-licensed projects but do not include source code. They are not produced by the Kivy Team, and we would always recommend caution before installing binaries from unknown sources.

Technical Questions

Can I write apps that support my local language?

The Kivy team is distributed globally and understands the importance of handling internationalization of software. They are actively working on a number of fronts to make Kivy support languages from around the world more seamlessly. It is challenging as Kivy is built on a number of technologies and platforms, with differing support for display and input of Unicode, Right-to-Left languages, etc.

This guide, Supporting Arabic Alphabet in Kivy for Building Cross-Platform Applications, provides helpful advice in handling many languages, not just Arabic.

For True Type fonts (TTF) on platforms using SDL/SDL2 for graphics, setting a Label's font_script_name correctly can avoid rendering issues with some fonts.

Should I make a property a Kivy class-level property?

Recall from the Kivy Properties documentation that the services provided by making something a Kivy Property are:

Value Checking / Validation: When you assign a new value to a property, the value is checked against validation constraints. For example, validation for an OptionProperty will make sure that the value is in a predefined list of possibilities. Validation for a NumericProperty will check that your value is a numeric type. This prevents many errors early on.
Observer Pattern: You can specify what should happen when a property’s value changes. You can bind your own function as a callback to changes of a Property. If, for example, you want a piece of code to be called when a widget’s pos property changes, you can bind a function to it.
Better Memory Management: The same instance of a property is shared across multiple widget instances.

Thus, if you think you will want to use any of those services in conjunction with an attribute, you should probably default to making that attribute a Kivy property. (Occasionally there may be a performance penalty in doing so, but generally speaking, it is advisable to err on the side of using what the Kivy devs have provided, and then later re-factor if such a performance hit becomes noticeable and detrimental. Just remember to avoid using Kivy "keywords," which vary somewhat from class to class, when re-factoring, lest you over-ride something unintentionally.)

Challenging Error messages

Here are some error messages that users have found difficult to debug.

I get a "Unable to get a Window, abort." error. What do I do?

If Kivy cannot instantiate a Window core provider (mostly SDL2), you'll see this. The underlying issue depends on many things:

Check your installation. Twice.
Check that your graphics driver support OpenGL 2.1 at the minimum. Otherwise, Kivy can't run.
If you use windows and ANGLE (KIVY_GL_BACKEND=angle_sdl2), check that you have DirectX 9 support.
If your platform doesn't support OpenGL, SDL2 cannot initialize OpenGL.
Don't mix the architecture of the dependencies (e.g. Python 64-bit and 32-bit extensions/SDL2)
Don't mix python installation: e.g. if you have Python and Anaconda installed, the Python actually run may be different than you think. Similarly, if you have multiple Python versions available on the PATH, they may clash.
Check your PATH to ensure that other programs in it do not provide the same dlls as Kivy/Python, or bad stuff can happen.
- This commonly happens if some other program that uses similar dependencies as Kivy adds itself to the PATH so that Kivy's dependencies clash with theirs.
- Please read this and this for more details on PATH.
- The best tool to troubleshoot this is with Dependency Walker explained here and here.
- But ensure that you're launching it from the identical environment that you start Python.
Ensure you have all dependencies installed (like kivy_deps.sdl2).
Maybe your drivers have some missing OpenGL symbols? Try to switch to another graphics backend with KIVY_GL_BACKEND.
Maybe your Pycharm configuration is incorrect.

I get a "Fatal Python error: (pygame parachute) Segmentation Fault" error. What went wrong?

Most of time, this issue is due to the usage of old graphics drivers. Install the latest graphics driver available for your graphics card, and it should be ok.

If not, this means you have probably triggered some OpenGL code without an available OpenGL context. If you are loading images, atlases, using graphics instructions, you must spawn a Window first:

method 1 (preferred)

from kivy.base import EventLoop
EventLoop.ensure_window()

method 2
```
from kivy.core.window import Window
```

If not, please make a bug report. Instructions are here. That kind of error can be very hard, so please give us all the information you can give about your environment and execution.

I get an "undefined symbol: glGenerateMipmap" error. How do I fix it?

Your graphics card or its drivers might be too old. Update your graphics drivers to the latest available version and retry.

I get an "ImportError: No module named _event". How come?

If you use Kivy from our development version, you must compile it before using it. Check installation guide for more information.

Android Questions

Android-related FAQ issues are documented in the Python-For-Android FAQ.

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