Re: The ** operator ambiguous?
Paul Boddie [EMAIL PROTECTED] wrote: However, this ambiguous usage of * and ** is one thing I don't recall appearing on any of the Python warts lists It is true that the same punctuation character is used in more than one context, but that is also true for many other punctuation characters. There is no ambiguity. -- http://mail.python.org/mailman/listinfo/python-list
The ** operator ambiguous?
I noticed that the ** operator is used as the power operator, however
I've seen it used when passing variables into a function. For example,
I was researching a way to combine dictionaries. I found that if you
do this:
a = {t1:a, t2:b}
b = {t3:c}
dict( a, **b )
This combines the two dictionaries. However, I have no idea what the
** operator is here. I know that when you specify ** as a parameter in
a function definition, it represents a dictionary of parameters passed
in. However, in this example it is NOT being used in a function
definition. It is being used when passing variables into a function.
Can someone explain what this means? I looked in the documentation but
I couldn't find anything.
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Re: The ** operator ambiguous?
Robert Dailey schrieb:
I noticed that the ** operator is used as the power operator, however
I've seen it used when passing variables into a function. For example,
I was researching a way to combine dictionaries. I found that if you
do this:
a = {t1:a, t2:b}
b = {t3:c}
dict( a, **b )
This combines the two dictionaries. However, I have no idea what the
** operator is here. I know that when you specify ** as a parameter in
a function definition, it represents a dictionary of parameters passed
in. However, in this example it is NOT being used in a function
definition. It is being used when passing variables into a function.
Can someone explain what this means? I looked in the documentation but
I couldn't find anything.
It's the opposite to the function definition **-operator:
def foo(a=0, b=1):
print a,b
foo(**{a:100, b:200})
- 100 200
Diez
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Re: The ** operator ambiguous?
Robert Dailey [EMAIL PROTECTED] wrote: However, I have no idea what the ** operator is here. I know that when you specify ** as a parameter in a function definition, it represents a dictionary of parameters passed in. However, in this example it is NOT being used in a function definition. It is being used when passing variables into a function. Can someone explain what this means? I looked in the documentation but I couldn't find anything. It is in the documentation. Look in the reference manual section 5.3.4 Calls (http://docs.python.org/ref/calls.html): If the syntax *expression appears in the function call, expression must evaluate to a sequence. Elements from this sequence are treated as if they were additional positional arguments; if there are postional arguments x1,...,xN , and expression evaluates to a sequence y1,...,yM, this is equivalent to a call with M+N positional arguments x1,...,xN,y1,...,yM. A consequence of this is that although the *expression syntax appears after any keyword arguments, it is processed before the keyword arguments (and the **expression argument, if any - see below). So: def f(a, b): ... print a, b ... f(b=1, *(2,)) 2 1 f(a=1, *(2,)) Traceback (most recent call last): File stdin, line 1, in ? TypeError: f() got multiple values for keyword argument 'a' f(1, *(2,)) 1 2 It is unusual for both keyword arguments and the *expression syntax to be used in the same call, so in practice this confusion does not arise. If the syntax **expression appears in the function call, expression must evaluate to a (subclass of) dictionary, the contents of which are treated as additional keyword arguments. In the case of a keyword appearing in both expression and as an explicit keyword argument, a TypeError exception is raised. -- http://mail.python.org/mailman/listinfo/python-list
Re: The ** operator ambiguous?
En Mon, 16 Jul 2007 14:40:14 -0300, Robert Dailey [EMAIL PROTECTED]
escribió:
I noticed that the ** operator is used as the power operator, however
I've seen it used when passing variables into a function. For example,
I was researching a way to combine dictionaries. I found that if you
do this:
a = {t1:a, t2:b}
b = {t3:c}
dict( a, **b )
This combines the two dictionaries. However, I have no idea what the
** operator is here. I know that when you specify ** as a parameter in
a function definition, it represents a dictionary of parameters passed
in. However, in this example it is NOT being used in a function
definition. It is being used when passing variables into a function.
Can someone explain what this means? I looked in the documentation but
I couldn't find anything.
See this section in the tutorial
http://docs.python.org/tut/node6.html#SECTION00670.
If you want the dirty details: http://docs.python.org/ref/calls.html
--
Gabriel Genellina
--
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Re: The ** operator ambiguous?
On Jul 16, 10:40 am, Robert Dailey [EMAIL PROTECTED] wrote:
I noticed that the ** operator is used as the power operator, however
I've seen it used when passing variables into a function. For example,
I was researching a way to combine dictionaries. I found that if you
do this:
a = {t1:a, t2:b}
b = {t3:c}
dict( a, **b )
This combines the two dictionaries.
Use dict.update to combine dictionaries.
-Mike
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Re: The ** operator ambiguous?
Robert Dailey wrote: I noticed that the ** operator is used as the power operator, however I've seen it used when passing variables into a function. Others have already pointed out the relevant documentation. However, this ambiguous usage of * and ** is one thing I don't recall appearing on any of the Python warts lists - not that I spend too much time following such matters. I imagine that * was initially chosen in order to be similar to the way one may handle collections of values in C function signatures (ie. using pointers), and that ** was merely a convenient next step as opposed to being analogous to the pointer to pointer notation from C. The same symbols are used in different ways in C and C++, of course. It's interesting to see a fresh interpretation of the notation, though. Paul -- http://mail.python.org/mailman/listinfo/python-list
