Add row if attribute doesn't exist in feature class (ArcGIS/ArcPy)

Question

I have 150 feature classes in a file geodatabase. Each consist of a few polygons with a different label class. I want to make a selection on Label "1E-06", but some feature classes do not have this label, only for instance label 1E-01 to 1E-05. Is it possible to make some if statement, so if label 1E-06 does not exist in feature class, add row with label = "1E-06". It doesnt have to have geometry. I want to merge all 1e-06 labels later. For statistics purposes I need the empty ones too.

EDIT: I tried it myself (with help of answer below), but still get stuck. Right now, it can find all the shapefiles and put it in a database. It can search for rows with my expression, but I want to add a row for all features without the expression. I tried this with for row not in scursor: but this gives an error. How can I add a row in the features who do not have a row with the stated expression?

import arcpy
import os
count = 0
ws = "N:\Documents\Project\data"
dst654 = "N:\Documents\Project\Contour654.gdb"
dst800 = "N:\Documents\Project\Contour800.gdb"
expression654 = "Label = '1E-06 654'"
expression800 = "Label = '1E-06 800'"
for dirpath, dirnames, filenames in arcpy.da.Walk(ws,datatype="FeatureClass"):
    count += 1
    for file in filenames:
     filepath = os.path.join(dirpath,file)
     file_nice = file.replace('-', '_')
     file_nice = file_nice.replace(' ', '_')
     file_nice= file_nice.replace('.', '_')
     if '654' in file_nice:
         outpath1 = os.path.join(dst654,file_nice[:-4])
         arcpy.CopyFeatures_management(filepath,outpath1)
         arcpy.AddField_management(outpath1, "gid", "DOUBLE", "", "", "", "", "NULLABLE", "", "")
         arcpy.CalculateField_management(outpath1, "gid", count, "PYTHON", "")
         with arcpy.da.SearchCursor(outpath1, ['Shape', 'Label'],
                            where_clause=expression654) as scursor:
                                for row in scursor:
                                    print "yes"
                                    print outpath1
     else: 
         outpath2 = os.path.join(dst800,file_nice[:-4])
         arcpy.CopyFeatures_management(filepath,outpath2)
         arcpy.AddField_management(outpath2, "gid", "DOUBLE", "", "", "", "", "NULLABLE", "", "")
         arcpy.CalculateField_management(outpath2, "gid", count, "PYTHON", "")
         with arcpy.da.SearchCursor(outpath2, ['Shape', 'Label'],
                            where_clause=expression800) as scursor:
                                for row in scursor:
                                    print "yes"
                                    print outpath2

Answer 1

What you are asking could be done in ModelBuilder but is probably better done as a Python script because of the if-then-else logic.

Your code will need to:

1. Create a featurelayer from the featureclass
2. Attempt to select a row in the featurelayer where label equals "1E-06"
3. If a selection (get count) returns a zero then insert a new row with your data
4. Move on to your new featureclass and start from step 1.

So look at the help file for insert cursors, and select by attribute. If you actually bother to scroll down the page all tools have code samples, it is a great way to learn arcpy using Python for free.