How to make this simple string function "pythonic"

Question

Coming from the C/C++ world and being a Python newb, I wrote this simple string function that takes an input string (guaranteed to be ASCII) and returns the last four characters. If there’s less than four characters, I want to fill the leading positions with the letter ‘A'. (this was not an exercise, but a valuable part of another complex function)

There are dozens of methods of doing this, from brute force, to simple, to elegant. My approach below, while functional, didn’t seem "Pythonic".

NOTE: I’m presently using Python 2.6 — and performance is NOT an issue. The input strings are short (2-8 characters), and I call this function only a few thousand times.

def copyFourTrailingChars(src_str):

    four_char_array = bytearray("AAAA")
    xfrPos = 4

    for x in src_str[::-1]:
        xfrPos -= 1
        four_char_array[xfrPos] = x
        if xfrPos == 0:
            break

    return str(four_char_array)


input_str = "7654321"
print("The output of {0} is {1}".format(input_str, copyFourTrailingChars(input_str)))

input_str = "21"
print("The output of {0} is {1}".format(input_str, copyFourTrailingChars(input_str)))

The output is:

The output of 7654321 is 4321
The output of 21 is AA21

Suggestions from Pythoneers?

Anand S Kumar · Accepted Answer · 2015-09-19 15:58:54Z

27

I would use simple slicing and then str.rjust() to right justify the result using A as fillchar . Example -

def copy_four(s):
    return s[-4:].rjust(4,'A')

Demo -

>>> copy_four('21')
'AA21'
>>> copy_four('1233423')
'3423'

edited Sep 19, 2015 at 15:58

answered Sep 16, 2015 at 6:08

Anand S Kumar

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4 Comments

SMGreenfield Over a year ago

@VigneshKalai - Wow! That is amazing! Sure am glad I asked!

The6thSense Over a year ago

@SMGreenfield I did not answer that it was anand s kumar :)

kmario23 Over a year ago

@anand : you can also pass the string directly as a parameter, which makes it more intuitive.

Anand S Kumar Over a year ago

@kmario23 I guessed you are talking about the Demo , yea you are correct there. Thank you, fixed it.

Alfred Huang · Accepted Answer · 2015-09-16 06:30:32Z

5

You can simple adding four sentinel 'A' character before the original string, then take the ending four characters:

def copy_four(s):
    return ('AAAA'+s)[-4:]

That's simple enough!

answered Sep 16, 2015 at 6:30

Alfred Huang

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1 Comment

SMGreenfield Over a year ago

I LOVE this -- so clever -- totally out-of-the-box thinking!

TigerhawkT3 · Accepted Answer · 2015-09-16 07:12:11Z

4

How about something with string formatting?

def copy_four(s):
    return '{}{}{}{}'.format(*('A'*(4-len(s[-4:])) + s[-4:]))

Result:

>>> copy_four('abcde')
'bcde'
>>> copy_four('abc')
'Aabc'

Here's a nicer, more canonical option:

def copy_four(s):
    return '{:A>4}'.format(s[-4:])

Result:

>>> copy_four('abcde')
'bcde'
>>> copy_four('abc')
'Aabc'

edited Sep 16, 2015 at 7:12

answered Sep 16, 2015 at 6:06

TigerhawkT3

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2 Comments

SMGreenfield Over a year ago

your canonical version looks real cool -- but being a Python newb I'm having a hard time understanding the syntactical trick(s) you used, particularly {:A>4}. Can you describe what you've done here?

TigerhawkT3 Over a year ago

@SMGreenfield - that right-aligns > the sliced string s[-4:] in a field {} with a maximum length of 4 characters, and fills in any strings that are too short with the character A.

Sveltely · Accepted Answer · 2015-09-16 06:11:34Z

3

You could use slicing to get the last 4 characters, then string repetition (* operator) and concatenation (+ operator) as below:

def trailing_four(s):
    s = s[-4:]
    s = 'A' * (4 - len(s)) + s
    return s

answered Sep 16, 2015 at 6:11

Sveltely

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Comments

Anurag Sharma · Accepted Answer · 2015-09-16 06:12:31Z

3

You can try this

def copy_four_trailing_chars(input_string)
  list_a = ['A','A','A','A']
  str1 = input_string[:-4]
  if len(str1) < 4:
    str1 = "%s%s" % (''.join(list_a[:4-len(str1)]), str1)
  return str1

answered Sep 16, 2015 at 6:12

Anurag Sharma

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