Maximum Line Length¶

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There are also some PEP-8 cheat sheets that can be also used. I found a good one here. It summarizes the most important points of the PEP-8 guideline. If you want to learn more about PEP-8, then you can read the full documentation. Visit also pep8.org. The website contains same information as the documentation, but it has been nicely formatted.

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 characterlines; plus, limiting windows to 80 characters makes it possible tohave several windows side-by-side. The default wrapping on suchdevices disrupts the visual structure of the code, making it moredifficult to understand. Therefore, please limit all lines to amaximum of 79 characters. For flowing long blocks of text (docstringsor comments), limiting the length to 72 characters is recommended.

The preferred way of wrapping long lines is by using Python’s impliedline continuation inside parentheses, brackets and braces. Long linescan be broken over multiple lines by wrapping expressions inparentheses. These should be used in preference to using a backslashfor line continuation.

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Backslashes may still be appropriate at times. For example, long,multiple with-statements cannot use implicit continuation, sobackslashes are acceptable:

Another such case is with assert statements.

Make sure to indent the continued line appropriately. The preferredplace to break around a binary operator is after the operator, notbefore it. Some examples:

How to convert Py2 code to Py2/3 code using futurize:

Step 0: setup¶

Step 0 goal: set up and see the tests passing on Python 2 and failing on Python 3.

1. Clone the package from github/bitbucket. Optionally rename your repo to package-future. Examples: reportlab-future, paramiko-future, mezzanine-future.

2. Create and activate a Python 2 conda environment or virtualenv. Install the package with pythonsetup.pyinstall and run its test suite on Py2.7 or Py2.6 (e.g. pythonsetup.pytest or py.test or nosetests)

3. Optionally: if there is a .travis.yml file, add Python version 3.3 and remove any versions < 2.6.

4. Install Python 3.3 with e.g. sudoapt-getinstallpython3. On other platforms, an easy way is to use Miniconda. Then e.g.:

Step 1: modern Py2 code¶

The goal for this step is to modernize the Python 2 code without introducing any dependencies (on future or e.g. six) at this stage.

1a. Install future into the virtualenv using:

1b. Run futurize--stage1-w*.pysubdir1/*.pysubdir2/*.py. Note that withrecursive globbing in bash or zsh, you can apply stage 1 to all Pythonsource files recursively with:

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1c. Commit all changes

1d. Re-run the test suite on Py2 and fix any errors.

See Stage 1: “safe” fixes for more info.

Example error¶

One relatively common error after conversion is:

If you get this error, try adding an empty __init__.py file in the packagedirectory. (In this example, in the tests/ directory.) If this doesn’t help,and if this message appears for all tests, they must be invoked differently(from the cmd line or e.g. setup.py). The way to run a module inside apackage on Python 3, or on Python 2 with absolute_import in effect, is:

(For more info, see PEP 328 andthe PEP 8 section on absoluteimports.)

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Step 2: working Py3 code that still supports Py2¶

The goal for this step is to get the tests passing first on Py3 and then on Py2again with the help of the future package.

2a. Run:

Or, using recursive globbing with bash or zsh, you can view the stage 2changes to all Python source files recursively with:

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To apply the changes, add the -w argument.

This stage makes further conversions needed to support both Python 2 and 3.These will likely require imports from future on Py2 (and sometimes on Py3),such as:

Optionally, you can use the --unicode-literals flag to add this import tothe top of each module:

All strings in the module would then be unicode on Py2 (as on Py3) unlessexplicitly marked with a b' prefix.

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If you would like futurize to import all the changed builtins to have theirPython 3 semantics on Python 2, invoke it like this:

2b. Re-run your tests on Py3 now. Make changes until your tests pass on Python 3.

2c. Commit your changes! :)

2d. Now run your tests on Python 2 and notice the errors. Add wrappers fromfuture to re-enable Python 2 compatibility. See theCheat Sheet: Writing Python 2-3 compatible code cheat sheet and What else you need to know for more info.

After each change, re-run the tests on Py3 and Py2 to ensure they pass on both.

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2e. You’re done! Celebrate! Push your code and announce to the world! Hashtags#python3 #python-future.