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1 | Writing code for Python 2 and 3 | |
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2 | =============================== | |
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3 | ||
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4 | Iterators | |
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5 | --------- | |
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6 | ||
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7 | Many built in functions and methods in Python 2 come in pairs, one | |
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8 | returning a list, and one returning an iterator (e.g. :func:`range` and | |
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9 | :func:`xrange`). In Python 3, there is usually only the iterator form, | |
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10 | but it has the name which gives a list in Python 2 (e.g. :func:`range`). | |
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11 | ||
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12 | The way to write compatible code depends on what you need: | |
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13 | ||
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14 | * A list, e.g. for serialisation, or to test if something is in it. | |
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15 | * Iteration, but it will never be used for very many items, so efficiency | |
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16 | isn't especially important. | |
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17 | * Iteration over many items, where efficiency is important. | |
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18 | ||
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19 | ================ ================= ======================= | |
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20 | list iteration (small) iteration(large) | |
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21 | ================ ================= ======================= | |
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22 | list(range(n)) range(n) py3compat.xrange(n) | |
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23 | list(map(f, it)) map(f, it) -- | |
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24 | list(zip(a, b)) zip(a, b) -- | |
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25 | list(d.items()) d.items() py3compat.iteritems(d) | |
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26 | list(d.values()) d.values() py3compat.itervalues(d) | |
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27 | ================ ================= ======================= | |
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28 | ||
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29 | Iterating over a dictionary yields its keys, so there is rarely a need | |
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30 | to use :meth:`dict.keys` or :meth:`dict.iterkeys`. | |
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31 | ||
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32 | Avoid using :func:`map` to cause function side effects. This is more | |
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33 | clearly written with a simple for loop. | |
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34 | ||
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35 | Changed standard library locations | |
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36 | ---------------------------------- | |
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37 | ||
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38 | Several parts of the standard library have been renamed and moved. This | |
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39 | is a short list of things that we're using. A couple of them have names | |
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40 | in :mod:`IPython.utils.py3compat`, so you don't need both | |
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41 | imports in each module that uses them. | |
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42 | ||
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43 | ================== ============ =========== | |
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44 | Python 2 Python 3 py3compat | |
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45 | ================== ============ =========== | |
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46 | :func:`raw_input` input input | |
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47 | :mod:`__builtin__` builtins builtin_mod | |
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48 | :mod:`StringIO` io | |
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49 | :mod:`Queue` queue | |
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50 | :mod:`cPickle` pickle | |
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51 | :mod:`thread` _thread | |
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52 | :mod:`copy_reg` copyreg | |
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53 | :mod:`urlparse` urllib.parse | |
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54 | :mod:`repr` reprlib | |
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55 | :mod:`Tkinter` tkinter | |
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56 | :mod:`Cookie` http.cookie | |
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57 | :mod:`_winreg` winreg | |
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58 | ================== ============ =========== | |
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59 | ||
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60 | Be careful with StringIO: :class:`io.StringIO` is available in Python 2.7, | |
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61 | but it behaves differently from :class:`StringIO.StringIO`, and much of | |
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62 | our code assumes the use of the latter on Python 2. So a try/except on | |
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63 | the import may cause problems. | |
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64 | ||
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65 | Unicode | |
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66 | ------- | |
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67 | ||
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68 | Always be explicit about what is text (unicode) and what is bytes. | |
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69 | *Encoding* goes from unicode to bytes, and *decoding* goes from bytes | |
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70 | to unicode. | |
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71 | ||
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72 | To open files for reading or writing text, use :func:`io.open`, which is | |
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73 | the Python 3 builtin ``open`` function, available on Python 2 as well. | |
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74 | We almost always need to specify the encoding parameter, because the | |
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75 | default is platform dependent. | |
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76 | ||
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77 | Relative imports | |
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78 | ---------------- | |
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79 | ||
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80 | :: | |
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81 | ||
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82 | # This makes Python 2 behave like Python 3: | |
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83 | from __future__ import absolute_import | |
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84 | ||
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85 | import io # Imports the standard library io module | |
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86 | from . import io # Import the io module from the package | |
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87 | # containing the current module | |
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88 | from .io import foo # foo from the io module next to this module | |
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89 | from IPython.utils import io # This still works | |
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90 | ||
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91 | Print function | |
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92 | -------------- | |
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93 | ||
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94 | :: | |
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95 | ||
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96 | # Support the print function on Python 2: | |
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97 | from __future__ import print_function | |
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98 | ||
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99 | print(a, b) | |
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100 | print(foo, file=sys.stderr) | |
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101 | print(bar, baz, sep='\t', end='') | |
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102 | ||
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103 | Metaclasses | |
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104 | ----------- | |
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105 | ||
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106 | The syntax for declaring a class with a metaclass is different in | |
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107 | Python 2 and 3. In most cases, the helper function | |
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108 | :func:`~IPython.utils.py3compat.with_metaclass` (copied from the six | |
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109 | library) can be used like this:: | |
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110 | ||
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111 | class FormatterABC(with_metaclass(abc.ABCMeta, object)): | |
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112 | ... | |
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113 | ||
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114 | Combining inheritance between Qt and the traitlets system, however, does | |
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115 | not work with this. Instead, we do this:: | |
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116 | ||
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117 | class QtKernelClientMixin(MetaQObjectHasTraits('NewBase', (HasTraits, SuperQObject), {})): | |
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118 | ... | |
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119 | ||
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120 | This gives the new class a metaclass of :class:`~IPython.qt.util.MetaQObjectHasTraits`, | |
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121 | and the parent classes :class:`~IPython.utils.traitlets.HasTraits` and | |
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122 | :class:`~IPython.qt.util.SuperQObject`. |
@@ -0,0 +1,8 b'' | |||
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1 | Single codebase Python 3 support | |
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2 | -------------------------------- | |
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3 | ||
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4 | IPython previously supported Python 3 by running 2to3 during setup. We | |
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5 | have now switched to a single codebase which runs natively on Python 2.7 | |
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6 | and 3.3. | |
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7 | ||
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8 | For notes on how to maintain this, see :doc:`/development/pycompat`. |
@@ -46,6 +46,7 b' extensions = [' | |||
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46 | 46 | 'sphinx.ext.autodoc', |
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47 | 47 | 'sphinx.ext.doctest', |
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48 | 48 | 'sphinx.ext.inheritance_diagram', |
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49 | 'sphinx.ext.intersphinx', | |
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49 | 50 | 'IPython.sphinxext.ipython_console_highlighting', |
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50 | 51 | 'IPython.sphinxext.ipython_directive', |
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51 | 52 | 'numpydoc', # to preprocess docstrings |
@@ -178,6 +179,7 b' html_additional_pages = {' | |||
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178 | 179 | # Output file base name for HTML help builder. |
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179 | 180 | htmlhelp_basename = 'ipythondoc' |
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180 | 181 | |
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182 | intersphinx_mapping = {'http://docs.python.org/2/': None} | |
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181 | 183 | |
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182 | 184 | # Options for LaTeX output |
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183 | 185 | # ------------------------ |
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