Python NameError: name is not defined (related to having default input argument types)

问题

I have a problem with the fact that I am calling len(myByteArray) in the input arguments to a function I am declaring. I'd like that to be a default argument, but Python doesn't seem to like it. myByteArray is of type bytearray. See documentation on bytearray here. I am accessing its built-in find function, documented here (see "bytes.find").

My function:

def circularFind(myByteArray, searchVal, start=0, end=len(myByteArray)):
    """
    Return the first-encountered index in bytearray where searchVal 
    is found, searching to the right, in incrementing-index order, and
    wrapping over the top and back to the beginning if index end < 
    index start
    """
    if (end >= start):
        return myByteArray.find(searchVal, start, end)
    else: #end < start, so search to highest index in bytearray, and then wrap around and search to "end" if nothing was found 
        index = myByteArray.find(searchVal, start, len(myByteArray))
        if (index == -1):
            #if searchVal not found yet, wrap around and keep searching 
            index = myByteArray.find(searchVal, 0, end)
        return index

Examples to attempt to use the function above:

#-------------------------------------------------------------------
#EXAMPLES:
#-------------------------------------------------------------------
#Only executute this block of code if running this module directly,
#*not* if importing it
#-see here: http://effbot.org/pyfaq/tutor-what-is-if-name-main-for.htm
if __name__ == "__main__": #if running this module as a stand-alone program
    import random
    random.seed(0)

    bytes = bytearray(100)
    for i in range(len(bytes)):
        bytes[i] = int(random.random()*256)

    print(list(bytes)); print();

    print('built-in method:')
    print(bytes.find(255))
    print(bytes.find(2,10,97))
    print(bytes.find(5,97,4))

    print('\ncircularFind:')
    print(circularFind(bytes,255))
    print(circularFind(bytes,2,10,97))
    print(circularFind(bytes,5,97,4))

Error:

NameError: name 'myByteArray' is not defined

If I just remove my default arguments (=0 and =len(myByteArray)), however, it works fine. But...I really want those default arguments there so that the start and end arguments are optional. What do I do?

In C++ this would be easy, as argument types are specified when you write functions.

回答1:

Python default arguments are evaluated when the function is defined. Rather, you want something like this:

def circularFind(myByteArray, searchVal, start=0, end=None):
    """
    Return the first-encountered index in bytearray where searchVal 
    is found, searching to the right, in incrementing-index order, and
    wrapping over the top and back to the beginning if index end < 
    index start
    """
    if end is None:
        end = len(myByteArray)
    # continue doing what you were doing

回答2:

At the time of passing the parameters, the parameters are not initialised

>>> def a(b=1,c=b):
...     print(c,b)
... 
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
NameError: name 'b' is not defined

so you need to send len of myByteArray as another variable.

So what you could do is,

def circularFind(myByteArray, searchVal, start=0, end=-1):
    if end == -1:
        end = len(myByteArray)
    #reset of code here.

来源：https://stackoverflow.com/questions/38986341/python-nameerror-name-is-not-defined-related-to-having-default-input-argument

标签

python

python-3.x

arguments

nameerror