script0.py

c = input("enter amount and currency with the sign RMB/EUR:")

if c[-3:-1i] in ['rmb','RMB','Rmb']:
  E=eval(c[0:-3])/7.8942
  print("your converted currency is {:.2f}EUR".format(E))
elif c[-3:-1]in['EUR','Eur','eur']:
  R=eval(c[0:-3])*7.8942
  print("your converted currency is {:.2f}RMB".format(R))
else:
  print("Format Error")

script1.md

This is the error we get:

$ python script0.py
  File "main.py", line 3
    if c[-3:-1i] in ['rmb','RMB','Rmb']:
              ^
SyntaxError: invalid syntax

Explanation: Looks like a spurious "i" that doesn't belong! Let's remove it.

script1.py

c = input("enter amount and currency with the sign RMB/EUR:")

if c[-3:-1] in ['rmb','RMB','Rmb']:
  E=eval(c[0:-3])/7.8942
  print("your converted currency is {:.2f}EUR".format(E))
elif c[-3:-1]in['EUR','Eur','eur']:
  R=eval(c[0:-3])*7.8942
  print("your converted currency is {:.2f}RMB".format(R))
else:
  print("Format Error")

script2.md

Now, there is no error when we run the program:

$ python script1.py
enter amount and currency with the sign RMB/EUR:341rmb
Format Error

However, the program doesn't recognize our format, even though the input 341rmb is supposed to be valid. Here's how to debug.

Tip: Determine which codepath your code should've run.

In this case, our first if condition should've triggered. This means we expect c[-3:-1] in ['rmb','RMB','Rmb'] to be True. This is the confusing part: our input definitely contains rmb. So, what is c[-3:-1] then?

Tip: Print values to check they match your expectations.

Let's print c[-3:-1] to check it.

script2.py

c = input("enter amount and currency with the sign RMB/EUR:")

print(c[-3:-1]) # added this line for debugging
if c[-3:-1] in ['rmb','RMB','Rmb']:
  E=eval(c[0:-3])/7.8942
  print("your converted currency is {:.2f}EUR".format(E))
elif c[-3:-1]in['EUR','Eur','eur']:
  R=eval(c[0:-3])*7.8942
  print("your converted currency is {:.2f}RMB".format(R))
else:
  print("Format Error")

script3.md

Let's run the program again, with the same input.

$ python script2.py
enter amount and currency with the sign RMB/EUR:341rmb
rm
Format Error

Whoa, c[-3:-1] is rm? Why doesn't it include the b? Here is a short series of tests in the interactive prompt to demonstrate why.

>>> test = "helloworld"
>>> test[:-1]  # chops off the last letter
'helloworl'
>>> test[-3:]  # keeps only the last 3 letters
'rld'
>>> test[-3:-1]  # keeps the second-to-last and third-to-last letters
'rl'

Ah, so now we know. We want the last 3 letters, so change this to c[-3:]. This is our final program.

script3.py

c = input("enter amount and currency with the sign RMB/EUR:")

if c[-3:] in ['rmb','RMB','Rmb']:
  E=eval(c[0:-3])/7.8942
  print("your converted currency is {:.2f}EUR".format(E))
elif c[-3:]in['EUR','Eur','eur']:
  R=eval(c[0:-3])*7.8942
  print("your converted currency is {:.2f}RMB".format(R))
else:
  print("Format Error")

script4.md

Tada! Your code works!

$ python script3.py
enter amount and currency with the sign RMB/EUR:134rmb
your converted currency is 16.97EUR
$ python script4.py
enter amount and currency with the sign RMB/EUR:134eur
your converted currency is 1057.82RMB