I struggled to get MQ in Python to work with Message properties when running on z/OS. I eventually found that mqinqmp() in 64 bit mode was not always working (31 bit mode was fine).
Also the documentation for mqinqmp() is not very clear.
You pass to mqinqmp(), a property name (or a generic like %), a buffer, the size of the buffer, the length of the data, and some other parameters (such as get first, or get next). It returns the property name, the property value ( possibly truncated) , and the true length of the property..
If your buffer is too small to contain all of the property, you get reason code MQRC_PROPERTY_VALUE_TOO_BIG, and should get the true length of the property, its name, type, and length.
This worked fine when it was a 31 bit program. When I ran it as a 64 bit program, it did not return the true length, and did not return the property name.
The documentation says
DataLength Type: MQLONG – output
This is the length in bytes of the actual property value
as returned in the Value area.
If DataLength is less than the property value length, DataLength is still filled in on return from the MQINQMP call. This allows the application to determine the size of the buffer required to accommodate the property value, and then reissue the call with a buffer of the appropriate size.
The DataLength is the actual length of the property. (The value returned in the Value area is truncated at Value Length).
What should happen
After you get the MQRC_PROPERTY_VALUE_TOO_BIG reason code, you reallocate the buffer with a size of DataLength and retry the request. It should work the second time.
For me the DataLength was 0, so every time around the loop it got MQRC_PROPERTY_VALUE_TOO_BIG.