Using .len() in IF THEN statement in ArcMap field calculator

Question

I have a string field with values of varying lengths (1-4). I need each value to have a length of 5 characters by adding zeros to the value until the length equals 5. For example:

123

needs to be:

00123

I am writing a Python function in ArcMap's Field Calculator but it is incorrect. This is my code:

def zeros(photoLen):
 if photoLen == 4:
 return "0"
 elif photoLen == 3:
 return "00"
 elif photoLen == 2:
 return "000"
 elif photoLen == 1:
 return "0000"
 else:
 return 0

And then I call the function like this:

zeros(!photoStr!)

What do I need to change so this function will work?

• Michael Stimson: You're passing a string so if len(photoLen)==4: is more correct, if photoLen == 4: will give a 'type mismatch' error comparing a string with a number and then return "0" + photoLen... but still, the answer by DWyne is how to right justify with a set number of 0's.

Answer 1

In addition to @DWynne's answer, I'll add that there is a built-in python function that does exactly this (located a little bit further down the page on DWynne's link). It's called zfill and according to the documentation:

Returns the numeric string left filled with zeros in a string of length width. A sign prefix is handled correctly. The original string is returned if width is less than or equal to len(s).

So, you can use this in your expression:

!photoStr!.zfill(5)

It's no better than using rjust, but it's a bit shorter. I use it all the time when I want to pad a string with zeroes.

• DWynne: I like this better than my answer
• Mike T: If there was an integer field called photo, just use str(!photo!).zfill(5)

Answer 2

Actually, you could do this without a function. Try using str.rjust. An expression of !photoStr!.rjust(5, '0') will return a string with padded 0's to the left of your number string.

• '1' becomes '00001'
• '12' becomes '00012'
• '123' becomes '00123'
• '1234' becomes '01234'

Answer 3

Python's builtin string formatting will do what you want:

'{0:0>5d}'.format(photoStr)

The 0 before the : means first argument (photoStr), the next 0 is the fill character,> means right-aligned, 5 is the length of the formatted string, then d means a decimal number.

For example:

>>> '{0:0>5d}'.format(123)
'00123'
>>> '{0:0>5d}'.format(13)
'00013'
>>> '{0:0>5d}'.format(1)
'00001'

Answer 4

These are great string functions that I didn't know about. Just for good measure, I'll add a method I've used in the past:

expression:

zeros(!photoStr!)

code block:

def zeros(value):
 desired_len = 5
 return (desired_len-len(value))*"0"+value

• Fezter: You could add the desired_len as a parameter to the function. Like def zeros(value, desired_len):. That way, you can change the length as required. Then, you call it like zeros(!photoStr!, 5) or zeros(!photoStr!, 8) etc.