Error in Python 3 when reading from a file?

Question

Python

I read lines from a file and output to the command line. At the same time, when I output what I read from the file, everything works, but when I perform small manipulations with this line, an error occurs. Why, after all, in both cases strings are displayed, the variable and is not displayed?
Mistake:

IndentationError: unindent does not match any outer indentation level

f = open('stroki.txt')
for line in f:
  print (line)
  a = "https://site.com/desc?target="+line+"&platform=message"
        print (a)

in the print (a) program at the same level, here I couldn’t get there, after posting the question it goes to the right

Answer 1

The reason was that opposite one line was indented with Tab, and the other with 4 spaces

Answer 2

because in python you need to indent strictly according to the nesting of blocks.
put
print (a)
on the same level as
a = " https://site.com/desc?target= "+line+"&platform=message"
and everything will be ok