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# Message Modification
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Limited capabilities for modifying existing GRIB messages and writing them to new files. Supports changing metadata keys and data values, but cannot create GRIB files from scratch.
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## Capabilities
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### Modifying Message Keys
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Change GRIB key values in existing messages.
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```python { .api }
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class gribmessage:
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def __setitem__(self, key: str, value):
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"""
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Set GRIB key values.
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Parameters:
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- key: str, GRIB key name
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- value: appropriate type for the key
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Common types: int, float, str, numpy.ndarray (for 'values')
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"""
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def __setattr__(self, name: str, value):
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"""Set GRIB keys as attributes (equivalent to __setitem__)"""
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```
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Usage example:
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```python
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grbs = pygrib.open('weather.grb')
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grb = grbs.readline()
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# Modify forecast time
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grb['forecastTime'] = 120
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# Or using attribute syntax
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grb.forecastTime = 120
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# Change parameter (e.g., to pressure tendency)
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grb['parameterNumber'] = 2
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grb['shortName'] = 'prate' # May change automatically
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# Modify level information
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grb['level'] = 850
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grb['typeOfLevel'] = 'isobaricInhPa'
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print(f"Modified message: {grb}")
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```
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### Modifying Data Values
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Change the actual meteorological data array in GRIB messages.
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```python { .api }
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class gribmessage:
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def __setitem__(self, key: str, value):
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"""
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For 'values' key, accepts numpy arrays to replace data.
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Parameters:
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- key: str, must be 'values' for data modification
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- value: numpy.ndarray, new data array
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Must be compatible with grid dimensions
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"""
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```
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Usage example:
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```python
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grbs = pygrib.open('weather.grb')
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grb = grbs.select(shortName='t', level=500)[0]
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# Get original data
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original_data = grb['values']
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print(f"Original range: {original_data.min():.1f} to {original_data.max():.1f}")
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# Apply transformations
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# Convert from Kelvin to Celsius
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celsius_data = original_data - 273.15
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# Apply smoothing filter
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from scipy import ndimage
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smoothed_data = ndimage.gaussian_filter(original_data, sigma=1.0)
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# Scale values
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scaled_data = original_data * 1.1
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# Set modified data back to message
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grb['values'] = celsius_data
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print(f"Modified range: {grb['values'].min():.1f} to {grb['values'].max():.1f}")
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# Update metadata to reflect change
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grb['units'] = 'C'
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grb['name'] = 'Temperature (Celsius)'
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```
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### Writing Modified Messages
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Convert modified messages to binary format and write to files.
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```python { .api }
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class gribmessage:
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def tostring(self) -> bytes:
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"""
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Encode message as binary GRIB string.
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Returns:
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bytes, binary GRIB message data ready for writing
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"""
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```
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Usage example:
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```python
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grbs = pygrib.open('input.grb')
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grb = grbs.readline()
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# Modify the message
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grb['forecastTime'] = 168
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grb['parameterNumber'] = 11 # Change to different parameter
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data = grb['values']
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grb['values'] = data * 0.95 # Scale data by 5%
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# Convert to binary format
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binary_msg = grb.tostring()
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# Write to new file
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with open('modified.grb', 'wb') as f:
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f.write(binary_msg)
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# Verify the written file
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new_grbs = pygrib.open('modified.grb')
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new_grb = new_grbs.readline()
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print(f"Written message: {new_grb}")
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print(f"Forecast time: {new_grb['forecastTime']}")
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new_grbs.close()
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```
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### Processing Multiple Messages
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Modify and write multiple messages to create new GRIB files.
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```python
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input_grbs = pygrib.open('input.grb')
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# Open output file for writing
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with open('processed.grb', 'wb') as output_file:
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# Process each message
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for grb in input_grbs:
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# Apply modifications based on parameter type
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if grb['shortName'] == 't': # Temperature
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# Convert K to C
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data = grb['values'] - 273.15
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grb['values'] = data
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grb['units'] = 'C'
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elif grb['shortName'] in ['u', 'v']: # Wind components
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# Scale wind speeds
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data = grb['values'] * 1.05
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grb['values'] = data
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# Extend forecast time by 6 hours
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grb['forecastTime'] = grb['forecastTime'] + 6
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# Write modified message
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binary_msg = grb.tostring()
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output_file.write(binary_msg)
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input_grbs.close()
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# Verify processed file
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processed_grbs = pygrib.open('processed.grb')
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print(f"Processed file contains {len(processed_grbs)} messages")
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processed_grbs.close()
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```
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### Message Reloading
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Reload original message data from file after modifications.
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```python { .api }
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def reload(grb):
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"""
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Reload gribmessage from file to refresh data.
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Creates new message object with original file data.
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Parameters:
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- grb: gribmessage, message to reload
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Returns:
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New gribmessage object with original data from file
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"""
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```
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Usage example:
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```python
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grbs = pygrib.open('weather.grb')
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grb = grbs.readline()
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# Store original values for comparison
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original_forecast_time = grb['forecastTime']
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original_data = grb['values'].copy()
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# Make modifications
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grb['forecastTime'] = 240
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grb['values'] = grb['values'] * 2.0
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print(f"Modified forecast time: {grb['forecastTime']}")
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print(f"Modified data range: {grb['values'].min():.1f} to {grb['values'].max():.1f}")
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# Reload original data
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original_grb = pygrib.reload(grb)
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print(f"Reloaded forecast time: {original_grb['forecastTime']}")
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print(f"Reloaded data range: {original_grb['values'].min():.1f} to {original_grb['values'].max():.1f}")
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```
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## Modification Limitations
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### Cannot Create New Files
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pygrib cannot create GRIB files from scratch - it can only modify existing messages.
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```python
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# NOT SUPPORTED: Creating new GRIB files from scratch
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# This will not work:
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# new_grb = pygrib.gribmessage() # Cannot instantiate directly
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# pygrib.create_grib_file() # No such function
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# SUPPORTED: Modify existing messages
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grbs = pygrib.open('existing.grb')
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grb = grbs.readline()
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# ... modify grb ...
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binary_data = grb.tostring() # Convert to binary for writing
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```
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### Key Modification Constraints
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Not all GRIB keys can be modified safely:
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```python
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grb = grbs.readline()
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# Safe modifications (common use cases)
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grb['forecastTime'] = 120 # Change forecast hour
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grb['level'] = 850 # Change level
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grb['values'] = new_data # Replace data array
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grb['units'] = 'new_unit' # Change units
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# Potentially problematic modifications
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# grb['Ni'] = 360 # Don't change grid dimensions
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# grb['Nj'] = 181 # without corresponding data changes
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# grb['gridType'] = 'lambert' # Don't change grid type
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# Always verify modifications work by testing the output
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try:
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binary_data = grb.tostring()
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print("Modification successful")
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except Exception as e:
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print(f"Modification failed: {e}")
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```
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### Data Array Compatibility
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When modifying data arrays, ensure compatibility with grid structure:
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```python
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grb = grbs.select(shortName='t', level=500)[0]
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original_data = grb['values']
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print(f"Original data shape: {original_data.shape}")
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print(f"Grid info: Ni={grb['Ni']}, Nj={grb['Nj']}")
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# New data must match grid dimensions
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if grb.expand_reduced:
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# For expanded grids, data shape should match (Nj, Ni)
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expected_shape = (grb['Nj'], grb['Ni'])
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else:
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# For reduced grids, data is 1D
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expected_shape = (grb['numberOfValues'],)
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# Create compatible new data
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new_data = original_data * 1.1 # Simple scaling preserves shape
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grb['values'] = new_data
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# Verify compatibility
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assert grb['values'].shape == original_data.shape
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```