pygrib

Warnings

• gotcha Installation can be complex due to the underlying ECCODES C library dependency. On some systems (especially Windows or when building from source), manual installation of ECCODES and setting environment variables like `ECCODES_DEFINITION_PATH` (e.g., to `$CONDA_PREFIX/Library/share/eccodes/definitions`) might be necessary to avoid errors like 'boot.def cannot be found'. Pip/conda generally attempt to handle this automatically, but issues can arise.
  Fix: Consult the official pygrib installation documentation or `eccodes` documentation for your specific operating system. Ensure `ECCODES_DEFINITION_PATH` is correctly set if you encounter file not found errors related to definitions.
• gotcha On Anaconda environments, compatibility issues with newer HDF5 library versions (e.g., 1.10.0) have caused `ImportError` due to linking problems with the `ecmwf_grib` dependency.
  Fix: When using `conda`, explicitly request an older HDF5 version, for example: `conda install pygrib hdf5=1.8`.
• gotcha Pygrib has limited capabilities for writing GRIB files. You can modify the contents (e.g., data values) of an *existing* GRIB message and write it, but you cannot create an entirely new GRIB file from scratch.
  Fix: If creating GRIB files from scratch is needed, consider alternative libraries or tools that offer full GRIB creation capabilities, or start by modifying a minimal template GRIB file.
• gotcha When modifying GRIB2 message values (`grb['values']`), the `orderOfSpatialDifferencing` key might be reset to a default (e.g., 0), which can lead to the output GRIB2 file being considered 'unusable' by other GRIB tools (like `wgrib2`).
  Fix: Before modifying `grb['values']`, store the original `grb['orderOfSpatialDifferencing']`. After setting new values, explicitly set `grb['orderOfSpatialDifferencing']` back to its original value: `original_order = grb['orderOfSpatialDifferencing']; grb['values'] = new_values; grb['orderOfSpatialDifferencing'] = original_order;`.
• breaking Compatibility issues have been reported with newer versions of Cython (e.g., > 0.29 and >= 3.1), which might lead to build failures if Cython is not pinned to a compatible version during installation.
  Fix: If building from source and encountering Cython errors, try pinning Cython to an older, compatible version (e.g., `pip install 'Cython<3.0'`) or ensure you are using the latest `pygrib` version that includes fixes for newer Cython releases.

Install

• pip install pygrib PyPI (recommended)
• conda install -c conda-forge pygrib Anaconda/Conda

Imports

• pygrib

  import pygrib

Quickstart

This quickstart demonstrates how to open a GRIB file, read a message, and extract key information such as data values, latitudes, and longitudes. Note that a valid GRIB file is required to run this example effectively. The example includes a placeholder for a GRIB file path and a warning if a dummy file is used.

import pygrib
import os

# Create a dummy GRIB file for demonstration purposes
# In a real scenario, you would have an actual GRIB file.
# This example can't create a GRIB file from scratch due to pygrib's limitations,
# so we simulate reading one.

# For actual usage, replace 'sample.grib2' with your file path.
# If you don't have one, consider downloading a small sample GRIB file from a meteorological data source.
# Example: A minimalist GRIB file (not functional for this code but demonstrates intent)

# For this example, assume 'sample.grib2' exists and contains at least one message.
# Placeholder for a real GRIB file path
grib_file_path = os.environ.get('GRIB_SAMPLE_PATH', 'sample.grib2')

# Create a dummy file if it doesn't exist for the example to run without FileNotFoundError
if not os.path.exists(grib_file_path):
    print(f"Warning: '{grib_file_path}' not found. Cannot run quickstart without a GRIB file.\n" \
          "Please set the GRIB_SAMPLE_PATH environment variable or create 'sample.grib2'.")
    # Optionally create a minimal (non-functional) placeholder to prevent immediate crash
    with open(grib_file_path, 'wb') as f:
        f.write(b'GRIB-DUMMY-FILE') # Not a valid GRIB file, but prevents FileNotFoundError


try:
    grbs = pygrib.open(grib_file_path)

    # Read the first message
    # .read() returns a list of messages, so we take the first element.
    grb = grbs.read(1)[0]

    print(f"GRIB Message 1: {grb}")
    print(f"Short Name: {grb.shortName}")
    print(f"Parameter Name: {grb.name}")
    print(f"Units: {grb.units}")
    print(f"Valid Date: {grb.validDate}")

    # Get data values (as a numpy array)
    data = grb.values
    print(f"Data shape: {data.shape}")
    print(f"Data min/max: {data.min()}/{data.max()}")

    # Get latitudes and longitudes
    lats, lons = grb.latlons()
    print(f"Latitudes shape: {lats.shape}, Longitudes shape: {lons.shape}")

    grbs.close()

except Exception as e:
    print(f"An error occurred: {e}")
    if 'not a GRIB file' in str(e):
        print("This often means the GRIB_SAMPLE_PATH points to an invalid or empty file.")
    if os.path.exists(grib_file_path) and os.path.getsize(grib_file_path) < 100: # heuristic for dummy file
        print("The quickstart created a dummy file; please provide a real GRIB file for full functionality.")