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Development Tools for python
The modules described in this chapter help you write software. For example, the pydoc
module takes a module and generates documentation based on the module’s contents. The doctest
and unittest
modules contains frameworks for writing unit tests that automatically exercise code and verify that the expected output is produced. 2to3 can translate Python 2.x source code into valid Python 3.x code.
The list of modules described in this chapter is:
typing
— Support for type hints- Type aliases
- NewType
- Callable
- Generics
- User-defined generic types
- The
Any
type - Nominal vs structural subtyping
- Classes, functions, and decorators
pydoc
— Documentation generator and online help systemdoctest
— Test interactive Python examples- Simple Usage: Checking Examples in Docstrings
- Simple Usage: Checking Examples in a Text File
- How It Works
- Which Docstrings Are Examined?
- How are Docstring Examples Recognized?
- What’s the Execution Context?
- What About Exceptions?
- Option Flags
- Directives
- Warnings
- Basic API
- Unittest API
- Advanced API
- DocTest Objects
- Example Objects
- DocTestFinder objects
- DocTestParser objects
- DocTestRunner objects
- OutputChecker objects
- Debugging
- Soapbox
unittest
— Unit testing framework- Basic example
- Command-Line Interface
- Command-line options
- Test Discovery
- Organizing test code
- Re-using old test code
- Skipping tests and expected failures
- Distinguishing test iterations using subtests
- Classes and functions
- Test cases
- Deprecated aliases
- Grouping tests
- Loading and running tests
- load_tests Protocol
- Test cases
- Class and Module Fixtures
- setUpClass and tearDownClass
- setUpModule and tearDownModule
- Signal Handling
unittest.mock
— mock object library- Quick Guide
- The Mock Class
- Calling
- Deleting Attributes
- Mock names and the name attribute
- Attaching Mocks as Attributes
- The patchers
- patch
- patch.object
- patch.dict
- patch.multiple
- patch methods: start and stop
- patch builtins
- TEST_PREFIX
- Nesting Patch Decorators
- Where to patch
- Patching Descriptors and Proxy Objects
- MagicMock and magic method support
- Mocking Magic Methods
- Magic Mock
- Helpers
- sentinel
- DEFAULT
- call
- create_autospec
- ANY
- FILTER_DIR
- mock_open
- Autospeccing
- Sealing mocks
unittest.mock
— getting started- Using Mock
- Mock Patching Methods
- Mock for Method Calls on an Object
- Mocking Classes
- Naming your mocks
- Tracking all Calls
- Setting Return Values and Attributes
- Raising exceptions with mocks
- Side effect functions and iterables
- Mocking asynchronous iterators
- Mocking asynchronous context manager
- Creating a Mock from an Existing Object
- Patch Decorators
- Further Examples
- Mocking chained calls
- Partial mocking
- Mocking a Generator Method
- Applying the same patch to every test method
- Mocking Unbound Methods
- Checking multiple calls with mock
- Coping with mutable arguments
- Nesting Patches
- Mocking a dictionary with MagicMock
- Mock subclasses and their attributes
- Mocking imports with patch.dict
- Tracking order of calls and less verbose call assertions
- More complex argument matching
- Using Mock
- 2to3 – Automated Python 2 to 3 code translation
- Using 2to3
- Fixers
lib2to3
– 2to3’s library
test
— Regression tests package for Python- Writing Unit Tests for the
test
package - Running tests using the command-line interface
- Writing Unit Tests for the
test.support
— Utilities for the Python test suitetest.support.script_helper
— Utilities for the Python execution tests
2to3 is a Python program that reads Python 2.x source code and applies a series of fixers to transform it into valid Python 3.x code. The standard library contains a rich set of fixers that will handle almost all code. 2to3 supporting library lib2to3
is, however, a flexible and generic library, so it is possible to write your own fixers for 2to3. lib2to3
could also be adapted to custom applications in which Python code needs to be edited automatically.
Using 2to3
2to3 will usually be installed with the Python interpreter as a script. It is also located in the Tools/scripts
directory of the Python root.
2to3’s basic arguments are a list of files or directories to transform. The directories are recursively traversed for Python sources.
Here is a sample Python 2.x source file, example.py
:
def greet(name): print "Hello, {0}!".format(name) print "What's your name?" name = raw_input() greet(name)
It can be converted to Python 3.x code via 2to3 on the command line:
$ 2to3 example.py
A diff against the original source file is printed. 2to3 can also write the needed modifications right back to the source file. (A backup of the original file is made unless -n
is also given.) Writing the changes back is enabled with the -w
flag:
$ 2to3 -w example.py
After transformation, example.py
looks like this:
def greet(name): print("Hello, {0}!".format(name)) print("What's your name?") name = input() greet(name)
Comments and exact indentation are preserved throughout the translation process.
By default, 2to3 runs a set of predefined fixers. The -l
flag lists all available fixers. An explicit set of fixers to run can be given with -f
. Likewise the -x
explicitly disables a fixer. The following example runs only the imports
and has_key
fixers:
$ 2to3 -f imports -f has_key example.py
This command runs every fixer except the apply
fixer:
$ 2to3 -x apply example.py
Some fixers are explicit, meaning they aren’t run by default and must be listed on the command line to be run. Here, in addition to the default fixers, the idioms
fixer is run:
$ 2to3 -f all -f idioms example.py
Notice how passing all
enables all default fixers.
Sometimes 2to3 will find a place in your source code that needs to be changed, but 2to3 cannot fix automatically. In this case, 2to3 will print a warning beneath the diff for a file. You should address the warning in order to have compliant 3.x code.
2to3 can also refactor doctests. To enable this mode, use the -d
flag. Note that only doctests will be refactored. This also doesn’t require the module to be valid Python. For example, doctest like examples in a reST document could also be refactored with this option.
The -v
option enables output of more information on the translation process.
Since some print statements can be parsed as function calls or statements, 2to3 cannot always read files containing the print function. When 2to3 detects the presence of the from __future__ import print_function
compiler directive, it modifies its internal grammar to interpret print()
as a function. This change can also be enabled manually with the -p
flag. Use -p
to run fixers on code that already has had its print statements converted.
The -o
or --output-dir
option allows specification of an alternate directory for processed output files to be written to. The -n
flag is required when using this as backup files do not make sense when not overwriting the input files.
New in version 3.2.3: The -o
option was added.
The -W
or --write-unchanged-files
flag tells 2to3 to always write output files even if no changes were required to the file. This is most useful with -o
so that an entire Python source tree is copied with translation from one directory to another. This option implies the -w
flag as it would not make sense otherwise.
New in version 3.2.3: The -W
flag was added.
The --add-suffix
option specifies a string to append to all output filenames. The -n
flag is required when specifying this as backups are not necessary when writing to different filenames. Example:
$ 2to3 -n -W --add-suffix=3 example.py
Will cause a converted file named example.py3
to be written.
New in version 3.2.3: The --add-suffix
option was added.
To translate an entire project from one directory tree to another use:
$ 2to3 --output-dir=python3-version/mycode -W -n python2-version/mycode
Fixers
Each step of transforming code is encapsulated in a fixer. The command 2to3 -l
lists them. As documented above, each can be turned on and off individually. They are described here in more detail.apply
Removes usage of apply()
. For example apply(function, *args, **kwargs)
is converted to function(*args, **kwargs)
.asserts
Replaces deprecated unittest
method names with the correct ones.
From | To |
---|---|
failUnlessEqual(a, b) |
assertEqual(a, b) |
assertEquals(a, b) |
assertEqual(a, b) |
failIfEqual(a, b) |
assertNotEqual(a, b) |
assertNotEquals(a, b) |
assertNotEqual(a, b) |
failUnless(a) |
assertTrue(a) |
assert_(a) |
assertTrue(a) |
failIf(a) |
assertFalse(a) |
failUnlessRaises(exc, cal) |
assertRaises(exc, cal) |
failUnlessAlmostEqual(a, b) |
assertAlmostEqual(a, b) |
assertAlmostEquals(a, b) |
assertAlmostEqual(a, b) |
failIfAlmostEqual(a, b) |
assertNotAlmostEqual(a, b) |
assertNotAlmostEquals(a, b) |
assertNotAlmostEqual(a, b) |
basestring
Converts basestring
to str
.buffer
Converts buffer
to memoryview
. This fixer is optional because the memoryview
API is similar but not exactly the same as that of buffer
.dict
Fixes dictionary iteration methods. dict.iteritems()
is converted to dict.items()
, dict.iterkeys()
to dict.keys()
, and dict.itervalues()
to dict.values()
. Similarly, dict.viewitems()
, dict.viewkeys()
and dict.viewvalues()
are converted respectively to dict.items()
, dict.keys()
and dict.values()
. It also wraps existing usages of dict.items()
, dict.keys()
, and dict.values()
in a call to list
.except
Converts except X, T
to except X as T
.exec
Converts the exec
statement to the exec()
function.execfile
Removes usage of execfile()
. The argument to execfile()
is wrapped in calls to open()
, compile()
, and exec()
.exitfunc
Changes assignment of sys.exitfunc
to use of the atexit
module.filter
Wraps filter()
usage in a list
call.funcattrs
Fixes function attributes that have been renamed. For example, my_function.func_closure
is converted to my_function.__closure__
.future
Removes from __future__ import new_feature
statements.getcwdu
Renames os.getcwdu()
to os.getcwd()
.has_key
Changes dict.has_key(key)
to key in dict
.idioms
This optional fixer performs several transformations that make Python code more idiomatic. Type comparisons like type(x) is SomeClass
and type(x) == SomeClass
are converted to isinstance(x, SomeClass)
. while 1
becomes while True
. This fixer also tries to make use of sorted()
in appropriate places. For example, this block
L = list(some_iterable) L.sort()
is changed to
L = sorted(some_iterable)
import
Detects sibling imports and converts them to relative imports.imports
Handles module renames in the standard library.imports2
Handles other modules renames in the standard library. It is separate from the imports
fixer only because of technical limitations.input
Converts input(prompt)
to eval(input(prompt))
.intern
Converts intern()
to sys.intern()
.isinstance
Fixes duplicate types in the second argument of isinstance()
. For example, isinstance(x, (int, int))
is converted to isinstance(x, int)
and isinstance(x, (int, float, int))
is converted to isinstance(x, (int, float))
.itertools_imports
Removes imports of itertools.ifilter()
, itertools.izip()
, and itertools.imap()
. Imports of itertools.ifilterfalse()
are also changed to itertools.filterfalse()
.itertools
Changes usage of itertools.ifilter()
, itertools.izip()
, and itertools.imap()
to their built-in equivalents. itertools.ifilterfalse()
is changed to itertools.filterfalse()
.long
Renames long
to int
.map
Wraps map()
in a list
call. It also changes map(None, x)
to list(x)
. Using from future_builtins import map
disables this fixer.metaclass
Converts the old metaclass syntax (__metaclass__ = Meta
in the class body) to the new (class X(metaclass=Meta)
).methodattrs
Fixes old method attribute names. For example, meth.im_func
is converted to meth.__func__
.ne
Converts the old not-equal syntax, <>
, to !=
.next
Converts the use of iterator’s next()
methods to the next()
function. It also renames next()
methods to __next__()
.nonzero
Renames __nonzero__()
to __bool__()
.numliterals
Converts octal literals into the new syntax.operator
Converts calls to various functions in the operator
module to other, but equivalent, function calls. When needed, the appropriate import
statements are added, e.g. import collections.abc
. The following mapping are made:
From | To |
---|---|
operator.isCallable(obj) |
callable(obj) |
operator.sequenceIncludes(obj) |
operator.contains(obj) |
operator.isSequenceType(obj) |
isinstance(obj, collections.abc.Sequence) |
operator.isMappingType(obj) |
isinstance(obj, collections.abc.Mapping) |
operator.isNumberType(obj) |
isinstance(obj, numbers.Number) |
operator.repeat(obj, n) |
operator.mul(obj, n) |
operator.irepeat(obj, n) |
operator.imul(obj, n) |
paren
Add extra parenthesis where they are required in list comprehensions. For example, [x for x in 1, 2]
becomes [x for x in (1, 2)]
.print
Converts the print
statement to the print()
function.raise
Converts raise E, V
to raise E(V)
, and raise E, V, T
to raise E(V).with_traceback(T)
. If E
is a tuple, the translation will be incorrect because substituting tuples for exceptions has been removed in 3.0.raw_input
Converts raw_input()
to input()
.reduce
Handles the move of reduce()
to functools.reduce()
.reload
Converts reload()
to importlib.reload()
.renames
Changes sys.maxint
to sys.maxsize
.repr
Replaces backtick repr with the repr()
function.set_literal
Replaces use of the set
constructor with set literals. This fixer is optional.standarderror
Renames StandardError
to Exception
.sys_exc
Changes the deprecated sys.exc_value
, sys.exc_type
, sys.exc_traceback
to use sys.exc_info()
.throw
Fixes the API change in generator’s throw()
method.tuple_params
Removes implicit tuple parameter unpacking. This fixer inserts temporary variables.types
Fixes code broken from the removal of some members in the types
module.unicode
Renames unicode
to str
.urllib
Handles the rename of urllib
and urllib2
to the urllib
package.ws_comma
Removes excess whitespace from comma separated items. This fixer is optional.xrange
Renames xrange()
to range()
and wraps existing range()
calls with list
.xreadlines
Changes for x in file.xreadlines()
to for x in file
.zip
Wraps zip()
usage in a list
call. This is disabled when from future_builtins import zip
appears.
lib2to3
– 2to3’s library
Source code: Lib/lib2to3/
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