What's the bad magic number error?

What's the "Bad magic number" ImportError in python, and how do I fix it?

The only thing I can find online suggests this is caused by compiling a .py -> .pyc file and then trying to use it with the wrong version of python. In my case, however, the file seems to import fine some times but not others, and I'm not sure why.

The information python's providing in the traceback isn't particularly helpful (which is why I was asking here...), but here it is in case it helps:

Traceback (most recent call last):
  File "run.py", line 7, in <module>
    from Normalization import Normalizer

Solution 1:

The magic number comes from UNIX-type systems where the first few bytes of a file held a marker indicating the file type.

Python puts a similar marker into its pyc files when it creates them.

Then the python interpreter makes sure this number is correct when loading it.

Anything that damages this magic number will cause your problem. This includes editing the pyc file or trying to run a pyc from a different version of python (usually later) than your interpreter.

If they are your pyc files, just delete them and let the interpreter re-compile the py files. On UNIX type systems, that could be something as simple as:

rm *.pyc

or:

find . -name '*.pyc' -delete

If they are not yours, you'll have to either get the py files for re-compilation, or an interpreter that can run the pyc files with that particular magic value.

One thing that might be causing the intermittent nature. The pyc that's causing the problem may only be imported under certain conditions. It's highly unlikely it would import sometimes. You should check the actual full stack trace when the import fails?

As an aside, the first word of all my 2.5.1(r251:54863) pyc files is 62131, 2.6.1(r261:67517) is 62161. The list of all magic numbers can be found in Python/import.c, reproduced here for completeness (current as at the time the answer was posted, it may have changed since then):

1.5:   20121
1.5.1: 20121
1.5.2: 20121
1.6:   50428
2.0:   50823
2.0.1: 50823
2.1:   60202
2.1.1: 60202
2.1.2: 60202
2.2:   60717
2.3a0: 62011
2.3a0: 62021
2.3a0: 62011
2.4a0: 62041
2.4a3: 62051
2.4b1: 62061
2.5a0: 62071
2.5a0: 62081
2.5a0: 62091
2.5a0: 62092
2.5b3: 62101
2.5b3: 62111
2.5c1: 62121
2.5c2: 62131
2.6a0: 62151
2.6a1: 62161
2.7a0: 62171

Solution 2:

Deleting all .pyc files will fix "Bad Magic Number" error.

find . -name "*.pyc" -delete

Solution 3:

Loading a python3 generated *.pyc file with python2 also causes this error.

Solution 4:

Take the pyc file to a windows machine. Use any Hex editor to open this pyc file. I used freeware 'HexEdit'. Now read hex value of first two bytes. In my case, these were 03 f3.

Open calc and convert its display mode to Programmer (Scientific in XP) to see Hex and Decimal conversion. Select "Hex" from Radio button. Enter values as second byte first and then the first byte i.e f303 Now click on "Dec" (Decimal) radio button. The value displayed is one which is correspond to the magic number aka version of python.

So, considering the table provided in earlier reply

• 1.5 => 20121 => 4E99 so files would have first byte as 99 and second as 4e
• 1.6 => 50428 => C4FC so files would have first byte as fc and second as c4