tell the interpreter to compile some semantics as the semantics which will be
available in the future Python version. In other words, Python uses
from __future__ import feature
to backport features from other higher Python versions to the current interpreter.
In Python 3, many features such as
print_function are already enabled, but
we still leave these future statements for backward compatibility.
Future statements are NOT import statements. Future statements change how
Python interprets the code. They MUST be at the top of the file. Otherwise,
Python interpreter will raise
If you’re interested in future statements and want to acquire more explanation, further information can be found on PEP 236 - Back to the __future__
List All New Features¶
__future__ is a Python
module. We can use it to check what kind of future features can import to
current Python interpreter. The fun is
import __future__ is NOT a future
statement, it is a import statement.
>>> from pprint import pprint >>> import __future__ >>> pprint(__future__.all_feature_names) ['nested_scopes', 'generators', 'division', 'absolute_import', 'with_statement', 'print_function', 'unicode_literals', 'barry_as_FLUFL', 'generator_stop', 'annotations']
Future statements not only change the behavior of the Python interpreter but
__future__._Feature into the current program.
>>> from __future__ import print_function >>> print_function _Feature((2, 6, 0, 'alpha', 2), (3, 0, 0, 'alpha', 0), 65536)
Replacing print statement to print function is one of the most
notorious decision in Python history. However, this change brings some
flexibilities to extend the ability of
>>> print "Hello World" # print is a statement Hello World >>> from __future__ import print_function >>> print "Hello World" File "<stdin>", line 1 print "Hello World" ^ SyntaxError: invalid syntax >>> print("Hello World") # print become a function Hello World
As print function, making text become Unicode is another infamous decision. Nevertheless, many modern programming languages’ text is Unicode. This change compels us to decode texts early in order to prevent runtime error after we run programs for a while. Further information can be found on PEP 3112.
>>> type("Guido") # string type is str in python2 <type 'str'> >>> from __future__ import unicode_literals >>> type("Guido") # string type become unicode <type 'unicode'>
Sometimes, it is counterintuitive when the division result is int or long.
In this case, Python 3 enables the true division by default. However, in
Python 2, we have to backport
division to the current interpreter. Further
information can be found on PEP 238.
>>> 1 / 2 0 >>> from __future__ import division >>> 1 / 2 # return a float (classic division) 0.5 >>> 1 // 2 # return a int (floor division) 0
Before Python 3.7, we cannot assign annotations in a class or a function if it is not available in the current scope. A common situation is the definition of a container class.
class Tree(object): def insert(self, tree: Tree): ...
$ python3 foo.py Traceback (most recent call last): File "foo.py", line 1, in <module> class Tree(object): File "foo.py", line 3, in Tree def insert(self, tree: Tree): ... NameError: name 'Tree' is not defined
In this case, the definition of the class is not available yet. Python interpreter cannot parse the annotation during their definition time. To solve this issue, Python uses string literals to replace the class.
class Tree(object): def insert(self, tree: 'Tree'): ...
After version 3.7, Python introduces the future statement,
perform postponed evaluation. It will become the default feature in Python 4.
For further information please refer to PEP 563.
from __future__ import annotations class Tree(object): def insert(self, tree: Tree): ...
New in Python 3.1
PEP 401 is just an Easter egg.
This feature brings the current interpreter back to the past. It enables the
<> in Python 3.
>>> 1 != 2 True >>> from __future__ import barry_as_FLUFL >>> 1 != 2 File "<stdin>", line 1 1 != 2 ^ SyntaxError: with Barry as BDFL, use '<>' instead of '!=' >>> 1 <> 2 True
braces is an Easter egg. The source code can be found on
>>> from __future__ import braces File "<stdin>", line 1 SyntaxError: not a chance