Python TypeError: can only concatenate str (not "bytes") to str

Question

I found a really old Python example (Mandelbrot) on my harddisk which worked years ago. But now I cannot fix it ... :-(

Traceback (most recent call last):
  File "/Volumes/SSD300/Python/Mandel.py", line 11, in <module>
    for X in j('BM'+P(M,v*x*3+26,26,12,v,x,1,24))or C:
TypeError: can only concatenate str (not "bytes") to str

Python 3.9.0 on macOS

_                                      =   (
                                        255,
                                      lambda
                               V       ,B,c
                             :c   and Y(V*V+B,B,  c
                               -1)if(abs(V)<6)else
               (              2+c-4*abs(V)**-0.4)/i
                 )  ;v,      x=1500,1000;C=range(v*x
                  );import  struct;P=struct.pack;M,\
            j  ='<QIIHHHH',open('M.bmp','wb').write
for X in j('BM'+P(M,v*x*3+26,26,12,v,x,1,24))or C:
            i  ,Y=_;j(P('BBB',*(lambda T:(T*80+T**9
                  *i-950*T  **99,T*70-880*T**18+701*
                 T  **9     ,T*i**(1-T**45*2)))(sum(
               [              Y(0,(A%3/3.+X%v+(X/v+
                               A/3/3.-x/2)/1j)*2.5
                             /x   -2.7,i)**2 for  \
                               A       in C
                                      [:9]])
                                        /9)
                                       )   )

struct.pack returned a bytes object, but this code attempted to concatenate it to the string 'BM', which is not possible — ForceBru
– ForceBru, Commented Dec 1, 2020 at 20:30

ShadowRanger · Accepted Answer · 2020-12-01 20:30:53Z

1

Presumably, P is returning a bytes object, while 'BM' remains a str. Simplest fix is to change 'BM' to b'BM', but given this code likely comes from Python 2, it probably expects to iterate over length 1 strings, not ints, so it might make sense to .decode('latin-1') the result of P to get a str that represents the raw bytes.

Porting from Python 2 to Python 3 when str was used in Python 2 in a way that's not entirely text or raw bytes oriented is tricky; this is one of those cases.

answered Dec 1, 2020 at 20:30

ShadowRanger

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