为什么我不能使用加星标的表达式? [英] Why can't I use a starred expression?
问题描述
我的代码
$ pythonPython 3.5.2 |Continuum Analytics, Inc.|(默认，2016 年 7 月 2 日，17:53:06)[GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] 在 linux 上输入帮助"、版权"、信用"或许可"以获取更多信息.>>>a = (1, 2)>>>'%d %d %d' % (0, *a)'0 1 2'>>>'%d %d %d' % (*a, 3)'1 2 3'>>>'%d %d' % (*a)文件<stdin>"，第 1 行语法错误:此处不能使用带星号的表达式>>>我的问题，为什么?
用更严肃的语气:我想要一个答案或参考，详细说明使用加星标的表达式的所有来龙去脉，因为碰巧我有时会对它的行为感到惊讶......
附录
为了反映一些启发性的评论，立即按照我的问题添加以下代码
<预><代码>>>>'%d %d' % (, *a)文件<stdin>"，第 1 行'%d %d' % (, *a)^语法错误:无效语法>>>'%d %d' % (*a,)'1 2'>>>(我在发布原始问题之前尝试了 (, a) 部分，但我已经省略了它，因为错误与主演无关.)
python ≥ 3.5 中有一种语法可以正常工作"，但我还是想了解一下.
这是因为:
(a)只是一个用括号括起来的值.它不是一个新的元组对象.所以你的表情:
<预><代码>>>>'%d %d' % (*a)将被翻译成:
<预><代码>>>>'%d %d' % * a这在python语法方面显然是错误的.
为了创建一个新的元组，用一个表达式作为初始化器，你需要在它后面添加一个',':
注意:除非 a 是一个生成器，在这种特殊情况下你可以只输入:
另外，如果我可以提出一些建议:您可以开始使用新样式的格式化表达式.他们很棒！
<预><代码>>>>"{} {}".format(*a)您可以在那些 two 段python 文档，还有这个 很棒的网站.上面的行使用了下面描述的参数解包机制.
更新:从 python 3.6 开始，您还可以使用字符串插值 - f-strings！这些在 PEP-498 中有描述，一些示例可以在 Python 文档.
加星标的表达式
带星号的表达式除了创建新的列表/元组/字典之外，还有更多用途.其中大部分都在本 PEP 中进行了描述，并且这个
所有这些都归结为两种:
R 值解包:
<预><代码>>>>a, *b, c = 范围(5)# a = 0# b = [1, 2, 3]# c = 4>>>10、*范围(2)(10, 0, 1)可迭代/字典对象初始化(请注意，您也可以解压缩列表中的字典！):
<预><代码>>>>[1, 2, *[3, 4], *[5], *(6, 7)][1, 2, 3, 4, 5, 6, 7]>>>(1, *[2, 3], *{a": 1})(1, 2, 3, 'a')>>>{a":1，**{b":2，c":3}，**{c":新 3"，d":4}}{'a':1，'b':2，'c':'新 3'，'d':4}当然，最常见的用法是参数解包:
positional_arguments = [12, 一个字符串", (1, 2, 3), other_object]关键字参数 = {主机名":本地主机"，端口":8080}发送(*positional_arguments，**keyword_arguments)这将转化为:
send(12, "a string", (1, 2, 3), other_object, hostname="localhost", port=8080)此主题已在另一个堆栈溢出问题中大量讨论.
My code
$ python
Python 3.5.2 |Continuum Analytics, Inc.| (default, Jul 2 2016, 17:53:06)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = (1, 2)
>>> '%d %d %d' % (0, *a)
'0 1 2'
>>> '%d %d %d' % (*a, 3)
'1 2 3'
>>> '%d %d' % (*a)
File "<stdin>", line 1
SyntaxError: can't use starred expression here
>>>
My question, why?
In a more serious tone: I'd like an answer, or a reference, that details all the ins and outs of using a starred expression, as it happens that I am sometimes surprised from its behaviours...
Addendum
To reflect some of the enlightening comments that immediately followed my question I add the following code
>>> '%d %d' % (, *a)
File "<stdin>", line 1
'%d %d' % (, *a)
^
SyntaxError: invalid syntax
>>> '%d %d' % (*a,)
'1 2'
>>>
(I had tried the (, a) part before posting the original question but I've omitted it 'cause the error was not related to the starring.)
There is a syntax, in python ≥ 3.5, that "just works" but nevertheless I would like some understanding.
It's because this:
(a)
Is just a value surrounded by parenthesis. It's not a new tuple object. So your expression:
>>> '%d %d' % (*a)
will get translated to:
>>> '%d %d' % * a
which is obviously wrong in terms of python syntax.
In order to create a new tuple, with one expression as an initializer, you need to add a ',' after it:
>>> '%d %d' % (*a,)
Note: unless a is a generator, in this particular situation you could just type:
>>> '%d %d' % a
Also, if I may suggest something: you could start using new-style formating expressions. They are great!
>>> "{} {}".format(*a)
You can read more about them in those two paragraphs of python documentation, also there is this great website. The line above uses argument unpacking mechanism described below.
Update: since python 3.6, you could also use string interpolation - f-strings! These are described in PEP-498, and some examples can be found in Python documentation.
Starred Expressions
There are many more uses to starred expression than just creating a new list/tuple/dictionary. Most of them are described in this PEP, and this one
All of them come down to two kinds:
R-value unpacking:
>>> a, *b, c = range(5)
# a = 0
# b = [1, 2, 3]
# c = 4
>>> 10, *range(2)
(10, 0, 1)
Iterable / dictionary object initialization (notice that you can unpack dictionaries inside lists too!):
>>> [1, 2, *[3, 4], *[5], *(6, 7)]
[1, 2, 3, 4, 5, 6, 7]
>>> (1, *[2, 3], *{"a": 1})
(1, 2, 3, 'a')
>>> {"a": 1, **{"b": 2, "c": 3}, **{"c": "new 3", "d": 4}}
{'a': 1, 'b': 2, 'c': 'new 3', 'd': 4}
Of course, the most often seen use is arguments unpacking:
positional_arguments = [12, "a string", (1, 2, 3), other_object]
keyword_arguments = {"hostname": "localhost", "port": 8080}
send(*positional_arguments, **keyword_arguments)
which would translate to this:
send(12, "a string", (1, 2, 3), other_object, hostname="localhost", port=8080)
This topic has already been covered to a substantial extent in another Stack Overflow question.
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