Using variable in Arcpy select analysis

Question

I need to use arcpy.Select_analysis() in my arcpy script, but I have problem with using variables. When I use hard coded values in where_clause, it is working.

where_clause = '"Area" > 20'
arcpy.Select_analysis("input", "output", where_clause)

When I try to use variables in where_clause because user of toolbox will enter name of field and value of minimimum area, there is always problem with invalid expression. I tried lots of types of expressions, but nothing is working.

field_area = "Area"
minimum_area = 20
where_clause = 'field_area > minimum_area'
arcpy.Select_analysis("input", "output", where_clause)

Error:

Runtime error Traceback (most recent call last): File "", line 1, in File "c:\program files (x86)\arcgis\desktop10.2\arcpy\arcpy\analysis.py", line 98, in Select raise e ExecuteError: ERROR 000358: Invalid expression field_area> minimum_area Failed to execute (Select).

I work in ArcMap 10.2.

• Richard Morgan: where_clause = '"' + field_area + '" > ' + str(minimum_area)
• zubro: It is still not working.Same error is there: ERROR 000358: Invalid expression "Area" > 20 Failed to execute (Select). But the expression in error message seems to be correct - "Area" > 20.
• Martin: You probably need to enclose the whole statement with '. Not tested, but try this: where_clause = '"' + field_area + '" > ' + "'" + str(minimum_area) + "'"
• Vince: Please edit the question to specify the data format being queried -- different query engines have different required fomats.
• Alex Tereshenkov: Don't mess up with those strings - use the arcpy field delimiters pro.arcgis.com/en/pro-app/arcpy/functions/… , sample here gis.stackexchange.com/a/149676/14435

Answer 1

The problem is that you're not using variables in your query. Instead you're using strings that happen to be the same as your variable names.

What you want is:

field_area = 'Area'
minimum_area = 20
where_clause = '{} > {}'.format(arcpy.AddFieldDelimiters('input', field_area), minimum_area)
arcpy.Select_analysis("input", "output", where_clause)

Of course, that's assuming that 'input' is the name of your feature class--as in your supposedly working example above. However, I would strongly recommend that you assign the feature class name to a variable instead:

field_area = 'Area'
minimum_area = 20
fcInput = 'ThePathAndOrNameOfMyFeatureClass'
fcOutput = 'ThePathAndOrNameOfTheOutputFeatureClass'
where_clause = '{} > {}'.format(arcpy.AddFieldDelimiters(fcInput, field_area), minimum_area)
arcpy.Select_analysis(fcInput, fcOutput, where_clause)

• Vince: +1 for using .format -- String math is too convoluted when true formatting is available. I wish I could give another plus-up for non-ambiguous variable naming.