一、概述
Python的每個新版本都會增加一些新的功能，或者對原來的功能作一些改動。有些改動是不兼容舊版本的，也就是在當前版本運行正常的代碼，到下一個版本運行就可能不正常了。
從Python 2.7到Python 3.x就有不兼容的一些改動，比如2.x里的字符串用'xxx'表示str，Unicode字符串用u'xxx'表示unicode，而在3.x中，所有字符串都被視為unicode。
因此，寫u'xxx'和'xxx'是完全一致的，而在2.x中以'xxx'表示的str就必須寫成b'xxx'，以此表示“二進制字符串”。
直接把代碼升級到3.x是比較冒進的，因為有大量的改動需要測試。相反，可以在2.7版本中先在一部分代碼中測試一些3.x的特性，如果沒有問題，再移植到3.x不遲。
Python提供了__future__模塊，把下一個新版本的特性導入到當前版本，於是我們就可以在當前版本中測試一些新版本的特性。
二、源代碼
個人Python環境，__future__路徑：C:\ProgramData\Anaconda3\Lib\__future__.py
"""Record of phased-in incompatible language changes.
Each line is of the form:
FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
CompilerFlag ")"
where, normally, OptionalRelease < MandatoryRelease, and both are 5-tuples
of the same form as sys.version_info:
(PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
PY_MINOR_VERSION, # the 1; an int
PY_MICRO_VERSION, # the 0; an int
PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
PY_RELEASE_SERIAL # the 3; an int
)
OptionalRelease records the first release in which
from __future__ import FeatureName
was accepted.
In the case of MandatoryReleases that have not yet occurred,
MandatoryRelease predicts the release in which the feature will become part
of the language.
Else MandatoryRelease records when the feature became part of the language;
in releases at or after that, modules no longer need
from __future__ import FeatureName
to use the feature in question, but may continue to use such imports.
MandatoryRelease may also be None, meaning that a planned feature got
dropped.
Instances of class _Feature have two corresponding methods,
.getOptionalRelease() and .getMandatoryRelease().
CompilerFlag is the (bitfield) flag that should be passed in the fourth
argument to the builtin function compile() to enable the feature in
dynamically compiled code. This flag is stored in the .compiler_flag
attribute on _Future instances. These values must match the appropriate
#defines of CO_xxx flags in Include/compile.h.
No feature line is ever to be deleted from this file.
"""
all_feature_names = [
"nested_scopes",
"generators",
"division",
"absolute_import",
"with_statement",
"print_function",
"unicode_literals",
"barry_as_FLUFL",
"generator_stop",
"annotations",
]
__all__ = ["all_feature_names"] + all_feature_names
# The CO_xxx symbols are defined here under the same names defined in
# code.h and used by compile.h, so that an editor search will find them here.
# However, they're not exported in __all__, because they don't really belong to
# this module.
CO_NESTED = 0x0010 # nested_scopes
CO_GENERATOR_ALLOWED = 0 # generators (obsolete, was 0x1000)
CO_FUTURE_DIVISION = 0x2000 # division
CO_FUTURE_ABSOLUTE_IMPORT = 0x4000 # perform absolute imports by default
CO_FUTURE_WITH_STATEMENT = 0x8000 # with statement
CO_FUTURE_PRINT_FUNCTION = 0x10000 # print function
CO_FUTURE_UNICODE_LITERALS = 0x20000 # unicode string literals
CO_FUTURE_BARRY_AS_BDFL = 0x40000
CO_FUTURE_GENERATOR_STOP = 0x80000 # StopIteration becomes RuntimeError in generators
CO_FUTURE_ANNOTATIONS = 0x100000 # annotations become strings at runtime
class _Feature:
def __init__(self, optionalRelease, mandatoryRelease, compiler_flag):
self.optional = optionalRelease
self.mandatory = mandatoryRelease
self.compiler_flag = compiler_flag
def getOptionalRelease(self):
"""Return first release in which this feature was recognized.
This is a 5-tuple, of the same form as sys.version_info.
"""
return self.optional
def getMandatoryRelease(self):
"""Return release in which this feature will become mandatory.
This is a 5-tuple, of the same form as sys.version_info, or, if
the feature was dropped, is None.
"""
return self.mandatory
def __repr__(self):
return "_Feature" + repr((self.optional,
self.mandatory,
self.compiler_flag))
nested_scopes = _Feature((2, 1, 0, "beta", 1),
(2, 2, 0, "alpha", 0),
CO_NESTED)
generators = _Feature((2, 2, 0, "alpha", 1),
(2, 3, 0, "final", 0),
CO_GENERATOR_ALLOWED)
division = _Feature((2, 2, 0, "alpha", 2),
(3, 0, 0, "alpha", 0),
CO_FUTURE_DIVISION)
absolute_import = _Feature((2, 5, 0, "alpha", 1),
(3, 0, 0, "alpha", 0),
CO_FUTURE_ABSOLUTE_IMPORT)
with_statement = _Feature((2, 5, 0, "alpha", 1),
(2, 6, 0, "alpha", 0),
CO_FUTURE_WITH_STATEMENT)
print_function = _Feature((2, 6, 0, "alpha", 2),
(3, 0, 0, "alpha", 0),
CO_FUTURE_PRINT_FUNCTION)
unicode_literals = _Feature((2, 6, 0, "alpha", 2),
(3, 0, 0, "alpha", 0),
CO_FUTURE_UNICODE_LITERALS)
barry_as_FLUFL = _Feature((3, 1, 0, "alpha", 2),
(3, 9, 0, "alpha", 0),
CO_FUTURE_BARRY_AS_BDFL)
generator_stop = _Feature((3, 5, 0, "beta", 1),
(3, 7, 0, "alpha", 0),
CO_FUTURE_GENERATOR_STOP)
annotations = _Feature((3, 7, 0, "beta", 1),
(4, 0, 0, "alpha", 0),
CO_FUTURE_ANNOTATIONS)
三、使用說明
1.division
- Python2
整數相除的結果為取整去余；浮點數相除為精確除法。
- Python3
默認為精確除法；如果需要取整時使用//，也稱之為地板除。
2.print
- Python2中print為打印關鍵字
- Python3中print為打印函數名
3.unicode_literals
- Python2，字符串默認為str類型；添加前綴u后變為Unicode類型
- Python3，字符串默認為Unicode類型；如果需要str類型，添加前綴b
4.absolute_import
首先需要了解相對導入、絕對導入的概念
相對導入：在不指明 package 名的情況下導入自己這個 package 的模塊，比如一個 package 下有 a.py 和 b.py 兩個文件，在 a.py 里 from . import b 即是相對導入 b.py
絕對導入：指明頂層 package 名，比如 import a，Python 會在 sys.path里尋找所有名為 a 的頂層模塊
- Python2，支持絕對導入，
- Python3，支持相對導入、絕對導入