topic Re: Remove prefixes in Python Questions

Remove prefixes

2Quiker — Mon, 08 Jan 2024 16:57:53 GMT

I am trying to remove prefixes from a table. I have the following code but it removes more than just street prefix. I my code I have E "space", "E ". I need to be able to strip the prefixes so I can sort, then delete duplicates. How can I remove just the prefixes?

with arcpy.da.UpdateCursor(table1,'STREET') as cursor: for row in cursor: #print row[0] if row[0].startswith("S "): #print "Deleting S" row [0] = row[0].lstrip('S ') elif row[0].startswith("E "): #print "Deleting E" row [0] = row[0].lstrip('E ') elif row[0].startswith("W "): #print "Deleting W" row [0] = row[0].lstrip('W ') elif row[0].startswith("N "): #print "Deleting N" row [0] = row[0].lstrip('N ') cursor.updateRow(row) del cursor

After code runs i get.

Before Code	After code
E Explorer	xplorer
E Expedition	xpedtion
E Exective	xecutive
E Exchange	xchange

Re: Remove prefixes

MErikReedAugusta — Mon, 08 Jan 2024 17:13:25 GMT

lstrip is treating "E " as being ["E", " "] and stripping all instances of those two characters from the left of the string, until it reaches a character other than those two.

Try this:

if (row[0].startswith('E ') or row[0].startswith('W ') or row[0].startswith('N ') or row[0].startswith('S ') ): # print('Deleting Cardinal') row[0] = row[0][2:] cursor.updateRow(row) # N.B. if you indent this INSIDE the if statement, # then it won't update any value it doesn't have # to which can be handy if you track edits, since # you didn't technically have anything to change # on the ones without a cardinal direction ### The ones below this are optional, if you have full-word cardinals on your prefixes. elif (row[0].startswith('East ') or row[0].startswith('West ') ): # print('Deleting Cardinal') row[0] = row[0][4:] elif (row[0].startswith('North ') or row[0].startswith('South ') ): # print('Deleting Cardinal') row[0] = row[0][5:]

Since you're always looking for a 2-character* substring at the beginning, you know if you found it that you only need the remainder of the substring after it. [2:] tells it to go to index 2 (the character after your space), and then just give you the rest of the string from there.

* The elif statements at line 15 & line 20 look for 4- and 5-character substrings, so the indices at lines 19 & 24 also change—assuming you need them.

EDIT: Unrelated sidenote, it is surprisingly a pain to edit code block statements for typos on this forum. Apologies to anyone who read it before I caught them.

Re: Remove prefixes

MErikReedAugusta — Mon, 08 Jan 2024 17:32:33 GMT

I'm too lazy to fix the typos in the codeblock right now, but it just occurred to me that the indices for the full-word ones are wrong, because I forgot to count the space.

Line 19:

row[0] = row[0][5:]

Line 24:

row[0] = row[0][6:]

Re: Remove prefixes

2Quiker — Mon, 08 Jan 2024 17:44:23 GMT

Thanks for the reply. I was coming back to my post to add code that worked for me.

try: # Update the street names in the table with arcpy.da.UpdateCursor(table, "STREET") as cursor: for row in cursor: street_name = row[0] # List of common street prefixes to be removed prefixes = ['N ', 'S ', 'E ', 'W ', 'North ', 'South ', 'East ', 'West '] # Remove prefixes from the street name for prefix in prefixes: if street_name.startswith(prefix): row[0] = street_name[len(prefix):].strip() cursor.updateRow(row) break

Your code did work tho. Again thanks for the reply!

Re: Remove prefixes

MErikReedAugusta — Mon, 08 Jan 2024 20:26:33 GMT

Efficient! I like it.

Just for fun, I tried to see if I could condense this further. Here it is!

prefixes = ['N ', 'S ', 'E ', 'W ', 'North ', 'South ', 'East ', 'West '] with arcpy.da.UpdateCursor(table, 'STREET') as cursor: for row in cursor: row[0] = min([row[0][len(prefix):] if row[0].startswith(prefix) else row[0] for prefix in prefixes], key=len) cursor.updateRow(row)

First, a list comprehension creates a list of your street name with every prefix attempted to be removed from it. Then, it turns out the min() function accepts a key argument. If you give it the built-in len, it gives you the shortest entry from that list back as a single item.

Since, by definition, every item in our generated list is either the original street name or that street name minus a prefix, the shortest possible will always* be the street name minus any applicable prefix.

*An important caveat here: "N South 5th Street" would return as "South 5th Street", which may or may not be desired. But then, that's a problem of all of the code in this thread.

Also, just to be cheeky, here it is even more condensed. I saved a whole 3 lines! But I haven't tested it. And while I think the logic and syntax are all sound, it's the height of absurdity, anyway. Please don't do this to whoever has to read your code behind you. 😛

with arcpy.da.UpdateCursor(table, 'STREET') as cursor: [cursor.updateRow(row) for [min([row[0][len(prefix):] if row[0].startswith(prefix) else row[0] for prefix in ['N ', 'S ', 'E ', 'W ', 'North ', 'South ', 'East ', 'West ']], key=len)] in cursor]

Re: Remove prefixes

JoshuaBixby — Tue, 09 Jan 2024 00:15:17 GMT

As fun as code golf can be, list comprehensions were proposed and accepted "to create lists" (PEP 202 – List Comprehensions | peps.python.org). I think many would argue that using a list comprehension to perform a mapping function isn't very Pythonic.

Re: Remove prefixes

JoshuaBixby — Tue, 09 Jan 2024 04:07:33 GMT

Another approach would be to use regular expressions. Although regular expressions might be a bit overkill for this specific situation, they are much more flexible to handle more complex situations:

The above will result in "N South 5th Street" becoming "5th Street". If you don't want double prefixes to be removed, just take the "+" out of the regular expression pattern.