Often Import related questions come up, for which I’m always wanting to inspect the proper import function used to see what Options and features I can get out of it.

For this I generally use this function:

findImportFormatFile[fmt_] :=  SelectFirst[   Map[Function @ FileNameJoin @ {#, fmt, "Import.m"},    System`ConvertersDump`$  FormatsDirectory    ],   FileExistsQ,   $  Failed   ] 

which for instance can find the Import definitions for "CSV":

findImportFormatFile["CSV"]  "/Applications/Mathematica.app/Contents/SystemFiles/Formats/CSV/Import.m" 

from which we can glean that the base import function is:

System`Convert`TableDump`ImportSV 

but in a fresh kernel this isn’t defined, even after we use System`ConvertersDump`LoadImportFormat.

Looking at the data further we see the issue is the "Sources" for the importer haven’t been loaded:

ImportExport`DefaultSources["Table"]  {"/Applications/Mathematica.app/Contents/SystemFiles/Kernel/SystemResources/MacOSX-x86-64/Convert/Table.mx"} 

And after we import this we’re good.

But I’d like a way to get all this data–import function, options names, "Sources", etc.–automatically.

How can I do this?

Helper code

Here’s a partial solution I wrote up before thinking to pose this question. Hopefully it’s helpful if someone wants to roll a scrape, if there’s no more legit way to do this.

getFormatImportDataRaw[fmt_] :=    Block[{$  Context =       "ItsASecretToEverybody`Private`", $  ContextPath = {"System`"}},    Replace[     SelectFirst[      Map[Function @ FileNameJoin @ {#, fmt, "Import.m"},       System`ConvertersDump`$  FormatsDirectory       ],      FileExistsQ,      $  Failed      ],     s_String :> s -> Import[s, "HeldExpressions"]     ]    ]; getFormatImportData[fmt_] :=  Module[{    data,     file    },   data = getFormatImportDataRaw[fmt];   If[data =!= $  Failed,    {file, data} = List @@ data;    data /. {      HoldPattern[Set[s_Symbol, v : _List | _Association]] :>       RuleCondition[Set[s, v], True]      };    With[{cont =        FirstCase[data, HoldPattern[Begin[c_]] :> c,         "System`FakeHo`", ∞]},     SortBy[        Switch[#[[1]],           "FormatName", 0,          "BinaryFormat", 2,          "File", 3,          _, 1          ] &        ]@       Flatten@{         "DocumentedElements" ->          FirstCase[data,            HoldPattern[             Set[_Symbol?(Function[Null,                  SymbolName[Unevaluated@#] == "$  DocumentedElements",                  HoldFirst]), v_]] :> Hold@v,           None,           ∞           ],         "HiddenElements" ->          FirstCase[data,            HoldPattern[             Set[_Symbol?(Function[Null,                  SymbolName[Unevaluated@#] == "$  HiddenElements",                  HoldFirst]), v_]] :> Hold@v,           None,           ∞           ],         FirstCase[data,          HoldPattern[            ImportExport`RegisterImport[             fmtName_,              raw_,             post : _List : {},             ruls___?System`ConvertersDump`RuleQ             ]            ] :>           {            "FormatName" -> fmtName,            Replace[             Flatten@List@raw,             {              (r : Rule | RuleDelayed)[k_, h_] :>               r[k, h],              f : Except[_Rule | _RuleDelayed] :> "DefaultImport" -> f              },             1             ],            Replace[             post,             {              (r : Rule | RuleDelayed)[k_, h_] :>               r[k, h],              f : Except[_Rule | _RuleDelayed] :> "DefaultImport" -> f              },             1             ],            Replace[{ruls},             ("AvailableElements" -> _Symbol) -> Nothing,             1             ],            "File" -> file            },          None,          ∞          ]         } /.       s_Symbol?(Function[Null,           Context[#] == "ItsASecretToEverybody`Private`",           HoldFirst]) :>        RuleCondition[        ToExpression[cont <> SymbolName[Unevaluated[s]], StandardForm,          Inactivate], True]     ],    {}    ]   ] 

It’s not great and if there’s a built-in way I’d much, much prefer that.

It does work for some formats, though:

getFormatImportData@"CSV" //    Take[#, 3] & // GeneralUtilities`PrettyForm  {     "FormatName" -> "CSV",      "DefaultElement" -> "Data",      "DefaultImport" -> (System`Convert`TableDump`ImportSV["CSV", System`Convert`TableDump`elem, All, SlotSequence[1]] &)  }