A few snippets for working with feature class fields.
The basic method of accessing fields in arcpy is arcpy.ListFields().
| import arcpy | |
| featureClass = "c:/path/Geodatabase.gdb/fc" | |
| # Get a list of field objects from a feature class | |
| fcFields = arcpy.ListFields(featureClass) | |
| # Print the field names, types and lengths using string formatting | |
| for field in fcFields: | |
| print("{0}: type = {1} length = {2}".format(field.name, field.type, field.length)) | |
| # Get a list of field names from a feature class using list comprehension | |
| fieldNames = [field.name for field in arcpy.ListFields(featureClass)] |
What is returned from arcpy.ListFields() is not field names, but a list of field objects which have accessible properties, including field name, type and length. These are printed above using string formatting. Getting a Python list of field names using list comprehension is straight out of the ArcGIS documentation. There are tons of great code samples there when you start sifting through it for specific tasks. I have learned to name the returned list of fields something like fieldList, and the returned list of names by naming it fieldNames so I can remember which data type I’m using.
Restricting the returned fields or field names
The following snippet illustrates ways you can restrict the returned fields: it’s always a good idea to limit the processing to what you actually need for the task at hand. For example, you can use a wildcard for field names in the middle parameter, or a field type in the third parameter. Another tidbit I found buried in a post by Drew Flater on an ArcGIS blog is how to prevent the return of required fields, such as OID or Shape, which can complicate things when using different data source types. Using list comprehension, he returns only those field names where the property “required” is false.
| fc = "c:/path/geodatabase.gdb/featureclass" | |
| # restrict returned fields by wildcard | |
| fieldList = arcpy.ListFields(fc,"OWN*") | |
| # restrict returned fields by field type | |
| fieldList = arcpy.ListFields(fc,"","String") | |
| # exclude required fields such as Shape and OID | |
| # (https://blogs.esri.com/esri/arcgis/2012/08/30/field-mapping-and-python-scripting) | |
| fieldNames = [field.name for field in arcpy.ListFields(fc) if not field.required] |
Using dictionaries to compare fields
I have used Python dictionaries to compare the field characteristics of two feature classes. For example, I tested for fields of the same name but different lengths before appending. When the fields are automatically mapped by having the same name, the append operation will fail if the source string field is longer than the destination string field.
In the snippet below, two dictionaries are created using the field names and lengths as the key:value pairs. This means we can access the fields by name. Iterating through the destination feature class dictionary, the script looks for key matches (in this case, field names) in the source dictionary. Then, it can access the length of the field in each dictionary by referencing the key (field name) and compare the two integers.
| import arcpy | |
| srcFC = "c:/path/Geodatabase.gdb/srcFc" | |
| destFC = "c:/path/Geodatabase.gdb/destFc" | |
| # get field lists from each feature class, restricting to "String" feature types | |
| srcFields = arcpy.ListFields(srcFC,"","String") | |
| destFields = arcpy.ListFields(destFC,"","String") | |
| # create empty dictionaries | |
| srcDict = {} | |
| destDict = {} | |
| # populate dictionaries with key value pairs of field name: field length | |
| # so that the properties can be accessed by field name | |
| for srcField in srcFields: | |
| srcDict[srcField.name] = [srcField.length] | |
| for destField in destFields: | |
| destDict[destField.name] = [destField.length] | |
| # iterate through destination dictionary looking | |
| # for matching fields in source | |
| # access field lengths in dictionary by field name | |
| # compare field lengths | |
| for fieldName, fieldLen in destDict.items(): | |
| if fieldName in srcDict: | |
| srcLen = srcDict[fieldName][0] | |
| destLen = destDict[fieldName][0] | |
| if destLen < srcLen: | |
| print("ERROR String field length mismatch") | |
| print(fieldName + " field is too long") | |
| else: | |
| print(fieldName + " all is good") |