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(NB: I could not find any good tags for this post. Please feel free to add better ones.)
Suppose that the file /some/path/dump.mx was generated during an earlier Mathematica session by evaluating an expression of the form
DumpSave["/some/path/dump.mx", ...]
...for some argument ... unknown to us.
Q: How can I find out which variables were set (or re-set) as a result of evaluating
Get["/some/path/dump.mx"];
...in my current Mathematica session?
Short summary
In your current Mathematica session, you can easily track creation of new symbols. There is however no simple way to track updating of symbols. As a workaround you can start a fresh kernel, load MX file there and track new symbols. This may give you some idea how given MX file will affect your current Mathematica session.
Methods to track new symbols defined by MX file can be found in the following question:
Update: I have moved some part of my answer from here to DumpSave for the forgetful, because that question is considered canonical.
Off[General::stop] you can show all messages for case 2, which probably makes that approach much more valuable...
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May 2, 2017 at 8:13
This answer doesn't help you with past uses of DumpSave to store variables, but rather it offers an alternative. One obvious way to store a variable to be retrieved later is to use Put, as in:
m = RandomReal[1, {1000,1000}];
Put[m, "matrix.m"]; //AbsoluteTiming
stored = Get["matrix.m"]; //AbsoluteTiming
m === stored
{2.22504, Null}
{1.40961, Null}
True
As the timing shows, this method is rather slow. It also has an issue where some objects change when run through this Put/Get round trip. An alternative that I like is to define new functions, PutMX and GetMX (note that this can be made more robust by using a package):
PutMX[expr_, file_] := Block[{res=expr}, DumpSave[file, res]]
GetMX[file_] := Block[{res}, Get[file]; res]
As you can see, PutMX and GetMX use DumpSave under the hood, but in such a way that they mimic Put and Get. As an example:
PutMX[m, "matrix.mx"]; //AbsoluteTiming
stored = GetMX["matrix.mx"]; //AbsoluteTiming
m === stored
{0.015008, Null}
{0.005111, Null}
True
There have been some earlier suggestions to use Export[file, expr, "MX"] and Import[file] instead, but this approach is quite a bit slower, due to overhead in the Import/Export framework. For instance:
Export["matrix2.mx", m, "MX"]; //AbsoluteTiming
stored = Import["matrix2.mx"]; //AbsoluteTiming
m === stored
{0.07974, Null}
{0.055974, Null}
True
As you can see, PutMX/GetMX are quite a bit faster.
PutMX will only be compatible with GetMX but not Import. So I looked up what Import/Export do with MX. They use the same method that you show here: assign to a symbol and DumpSave, but they also wrap the expression with HoldComplete. The symbol used is System`Private`ConvertersPrivateDumpSymbol. If you also use this symbol instead of res (and wrap with HoldComplete), you could make PutMX compatible with Import.
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Import/Export do not use Block. Instead, Export just sets a variable and leaves it like that to consume memory ... it gets cleared only when Import is used.
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Export in MX, the symbol System`Private`ConvertersPrivateDumpSymbol retains a copy of the exported variable in memory. This should probably be reported.
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May 1, 2017 at 9:01
Exportto the.mxformat. If you use this method, then only the data will be saved, but any definitions. I much prefer this way. I can simply re-Importand assign to whatever variable name I prefer. If I want to save several pieces of data, and even give them names, then I use an association (or rule list in earlier versions). $\endgroup$Export/Importdoes seem like the way to go. Thanks! $\endgroup$