Syntax error when concatenating a string in Cython

Question

I have the following code inside a cdef class (Cython language):

def toString(self):
    res = "lut= " + str(self._mm_np[0].lut) +
        "n1= "  + str(self._mm_np[0].n1) +
        "nlay= "+ str(self._mm_np[0].nlay) +
        "n3= "  + str(self._mm_np[0].n3)
    return res

when I try to compile the cython file containing this code I get the following syntax error: "Expected an identifier or literal" that pointing on the spot with the first '+' in string concatenation.

I have tried to use '\' instead of '+' with no success.. What is the right way to concatenate strings in Pyhton/Cython? Thank you!

Makoto · Accepted Answer · 2015-01-22 17:05:19Z

1

You're missing the line continuation operator \:

def toString(self):
    res = "lut= " + str(self._mm_np[0].lut) + \
    "n1= "  + str(self._mm_np[0].n1) + \
    "nlay= "+ str(self._mm_np[0].nlay) + \
    "n3= "  + str(self._mm_np[0].n3)
    return res

...but you really shouldn't do that. It's considered poor style.

Explore the usage of the .format method for strings instead; it will provide positional arguments to that string so you don't have to concatenate.

def toString(self): 
    return "lut={} n1={} nlay={} n3={}".format(
                str(self._mm_np[0].lut),
                str(self._mm_np[0].n1),
                str(self._mm_np[0].nlay),
                str(self._mm_np[0].n3))

answered Jan 22, 2015 at 17:05

Makoto

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