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Step-by-step guide

You can either use the API or the grib_dump tool for this.

The For the moment there is no simple way of knowing what the type of a key value is. However, the library provides a function that can be used to find out what the native type of a key value is.

In the C API, there Python interface there is a function called "codes_get_native_type" which returns an integer which can be checked against the following macros:

CODES_TYPE_UNDEFINED
CODES_TYPE_LONG
CODES_TYPE_DOUBLE
CODES_TYPE_STRING
CODES_TYPE_BYTES
CODES_TYPE_SECTION
CODES_TYPE_LABEL
CODES_TYPE_MISSING

...

one of: "int", "float" or "str". E.g.

gid = codes_grib_new_from_samples("GRIB2")
codes_get_native_type(gid, "date")           # returns int
codes_get_native_type(gid, "referenceValue") # returns float
codes_get_native_type

...

(gid, "stepType")       # returns str

As of version 2.14.0, one can use the grib_dump tool with the "-p" option to dump a given key and query its type with the "-t" option.
E.g.Let's look at the types of various keys in a GRIB2 message:

% grib_dump -t -p identifier my.grib
  # type ascii (str)
  #-READ ONLY- identifier = GRIB;

% grib_dump -t -p typeOfProcessedData my.grib
  # type codetable (int)
  # Analysis and forecast products (grib2/tables/4/1.4.table)  
  typeOfProcessedData = 2;

% grib_dump -t -p numberOfDataPoints my.grib
  # type unsigned (int)
  numberOfDataPoints = 496;

% grib_dump -t -p scaleFactorOfFirstFixedSurface my.grib
  # type signed (int)
  scaleFactorOfFirstFixedSurface = MISSING;

% grib_dump -t -p referenceValue my.grib
  # type ieeefloat (double)
  #-READ ONLY- referenceValue = 45.67;

...

• "ascii" means a string (character array)
• "codetable" means an integer whose values come from entries in the given Code Table e.g. 1.4
• "unsigned" is a non-negative integer
• "signed" means an integer which can be negative
• "ieeefloat" is a floating-point number (https://en.wikipedia.org/wiki/IEEE_754)

Note: The above command could be run passing in all the keys in one go:

% grib_dump -t -p identifier,typeOfProcessedData,numberOfDataPoints,scaleFactorOfFirstFixedSurface,referenceValue my.grib

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