in python how do I convert a single digit number into a double digits string?

In Python 3.6 you can use so called f-strings. In my opinion this method is much clearer to read.

>>> f'{a:02d}'
'05'

In Python3, you can:

print("%02d" % a)

Based on what @user225312 said, you can use .zfill() to add paddng to numbers converted to strings.

My approach is to leave number as a number until the moment you want to convert it into string:

>>> num = 11
>>> padding = 3
>>> print(str(num).zfill(padding))
011

Branching off of Mohommad's answer:

str_years = [x for x in range(24)]
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]

#Or, if you're starting with ints:
int_years = [int(x) for x in str_years]

#Formatted here
form_years = ["%02d" % x for x in int_years]

print(form_years)
#['00', '01', '02', '03', '04', '05', '06', '07', '08', '09', '10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20', '21', '22', '23']

df["col_name"].str.rjust(4,'0')#(length of string,'value') --> ValueXXX --> 0XXX  
df["col_name"].str.ljust(4,'0')#(length of string,'value') --> XXXValue --> XXX0

If you are an analyst and not a full stack guy, this might be more intuitive:

[(str('00000') + str(i))[-5:] for i in arange(100)]

breaking that down, you:

• start by creating a list that repeats 0's or X's, in this case, 100 long, i.e., arange(100)

• add the numbers you want to the string, in this case, numbers 0-99, i.e., 'i'

• keep only the right hand 5 digits, i.e., '[-5:]' for subsetting

• output is numbered list, all with 5 digits

This is a dumb solution but I was getting type errors with the other solutions above. So if all else fails, yolo:

images3digit = []

for i in images:
        if len(i)==1:
            i = '00'+i
            images3digit.append(i)
        elif len(i)==2:
            i = '0'+i
            images3digit.append(i)
        elif len(i)==3:
            images3digit.append(i)