genericSlice

gistfile1.py

def genericSlice(length,start=None,stop=None,step=None,includeStop=False,oneBased=False,checkBounds=False):
    '''A generic version of "slice" supporting:
       * dropping or keeping the stop value
       * 0-based and 1-based indexing
       * optional bounds checking
       * +/-/None indexes (as usual)
       
       Requires the length of the list that will be sliced.
       
       Returns a slice object.
       (failure results in a None value at the moment)'''
    maxStop = (length-1 if includeStop else length)
    
    # Always fail if step is 0
    if step==0:
        return None
    
    # Fail on 0-values in 1-based indexing when checking bounds
    if checkBounds and oneBased and (start==0 or stop==0):
        return None
    
    # Wrap negative values to positive ones and then convert from
    # 1-based to 0-based indexing
    if start!=None:
        if start<0:
            start += length
        elif oneBased:
            start -=1
    if stop!=None:
        if stop<0:
            stop += length
        elif oneBased:
            stop -=1
    
    # If either value goes negative and we are not checking bounds,
    # then set it to None avoid wrap-around
    if not checkBounds:
        if start<0:
            start=None
        if stop<0:
            stop=None
    
    # Handle all the None cases
    if step==None:
        step=1
    if step>0:
        if start==None:
            start = 0
        if stop==None:
            stop = maxStop
    else:
        if start==None:
            start = length-1
        if stop==None:
            stop = (0 if includeStop else -1)
    
    # Fail on out-of-bounds if checkBounds is specified (still somewhat lenient)
    if checkBounds:
        if ( not 0<=start<length or
             not 0<=stop<length or
             step>0 and start-stop>1 or
             step<0 and stop-start>1 ):
            return None
    
    # Increment (or decrement) the stop value to include it
    if includeStop:
        # Add the sign of the step to the stop
        stop += (1 if step>0 else -1)
        
    # If this made stop negative, set it to None avoid wrap-around
    if stop<0:
        stop=None
    
    return slice(start,stop,step)

l=[1,2,3,4,5]
length=len(l)
l[genericSlice(length,1,None,2)]
l[genericSlice(length,1,None,2,oneBased=True)]