Currency Converter in Python 3

Question

Beginner Python developer here. I was tinkering around with Python and decided to build a currency converter. It takes in input from the user and prints out the converted currency values. The fixer.io API is used to get currency rates. Looking for any improvements that I can make in it.

GitHub

Code

# importing required libraries
import requests
import pycountry
from _datetime import datetime
from babel import numbers


def error_sev():
    print("Sorry! There seems to be an error. Please check if your network is working")


def error_inp():
    print("Sorry! There seems to be an error. Please check if the currencies entered are valid.")


def currency_print(input_cur, output_cur, input_currency_name, output_currency_name, amount, rate):
    # printing out exchange rate
    print("The rate for {} to {} as on {} is: "
          .format(input_currency_name, output_currency_name, date.strftime("%d-%m-%Y")), end='')
    print(numbers.format_currency(1, input_cur, locale='en') + " = " +
          numbers.format_currency(rate, output_cur, locale='en'))

    # printing converted value
    print("\t", end='')
    print(numbers.format_currency(amount, input_cur, locale='en') + " = " +
          numbers.format_currency(amount * rate, output_cur, locale='en'))

    print('-'*100)


# list of available currencies
currencies = [
    'USD', 'JPY', 'BGN', 'CZK', 'DKK', 'GBP', 'HUF', 'PLN', 'RON', 'SEK', 'CHF', 'NOK', 'HRK', 'RUB', 'TRY',
    'AUD', 'BRL', 'CAD', 'CNY', 'HKD', 'IDR', 'ILS', 'INR', 'KRW', 'MXN', 'MYR', 'NZD', 'PHP', 'SGD', 'THB',
    'ZAR', 'ISK'
]

# printing the list of available currencies for the user
print("Available currencies: ", end='')
for item in sorted(currencies)[:-1]:
    print(item, end=', ')
print(sorted(currencies)[-1])

# taking user input
try:
    amount = float(input("Enter amount: "))
# checking for input errors
except ValueError:
    print("Invalid input. Please enter only numbers.")

else:
        # taking currency values as input from user
        input_cur = input("Enter base currency code: ").upper()
        output_cur = input("Enter desired currency code (leave blank for all currencies): ").upper()

        # if user has specified desired currency
        if output_cur != '':
            response_url = "http://api.fixer.io/latest?base={}&symbols={}".format(input_cur, output_cur)
            response = requests.get(response_url)
            # checking for validity of inputs

            if output_cur in currencies and input_cur in currencies:

                # checking for validity of server
                if response.status_code is 200:
                    # parsing JSON response
                    data = response.json()
                    date = datetime.strptime(data['date'], "%Y-%m-%d")

                    rate = data['rates'][output_cur]

                    print('-' * 100)

                    # Getting currency names
                    input_currency_name = pycountry.currencies.get(alpha_3=input_cur).name
                    output_currency_name = pycountry.currencies.get(alpha_3=output_cur).name

                    currency_print(input_cur, output_cur, input_currency_name, output_currency_name, amount, rate)
                else:
                    # printing a server error
                    error_sev()
            else:
                # printing an input error
                error_inp()

        # if user has not specified desired currency, print out all conversions
        else:
            response_url = "http://api.fixer.io/latest?base={}".format(input_cur)
            response = requests.get(response_url)

            # checking for input validity
            if input_cur in currencies:

                # checking for validity of inputs and server
                if response.status_code is 200:
                    # parsing JSON response
                    data = response.json()
                    date = datetime.strptime(data['date'], "%Y-%m-%d")

                    print('-' * 100)

                    # looping through all rates
                    rates = data['rates']
                    for rate in sorted(rates):

                        cur_rate = rates[rate]
                        input_currency_name = pycountry.currencies.get(alpha_3=input_cur).name
                        output_currency_name = pycountry.currencies.get(alpha_3=rate).name

                        print("{} ({})".format(output_currency_name, rate))
                        print("\t", end='')

                        currency_print(input_cur, rate, input_currency_name, output_currency_name, amount, cur_rate)

                else:
                    # printing a server error
                    error_sev()
            else:
                # printing an input error
                error_inp()

Answer 1

Code Style Improvements

• "Flat is better than nested". You can make an early exit in case of invalid input:

  import sys
  
  try:
      amount = float(input("Enter amount: "))
  except ValueError:
      print("Invalid input. Please enter only numbers.")
      sys.exit(1)
  

  That will allow you to remove the else: part and continue on the top-level. Or, you can let the user retry the input until it is valid

• on the same topic of decreasing nestedness depth - add more early exists. For instance, if input currencies are invalid, throw an error and exit. Then, remove the else: and continue with your "positive case" logic on the same level. This should improve overall readability
• define the constants, like the list of currencies, as per PEP8 - in upper case (reference)
• put the main execution logic to under if __name__ == '__main__':
• you can simplify if output_cur != '': with just if output_cur:
• I'm not sure why you are importing datetime from _datetime (with underscore). I would expect the import to be from datetime import datetime
• don't put comment for obvious parts of the code. For example, "importing required libraries" does not provide any useful information.

• organize imports per PEP8 - stdlib libraries, then a newline, third-parties, a new line and then your "local" dependencies, all sorted alphabetically:

  from datetime import datetime
  
  from babel import numbers
  import pycountry
  import requests
  

Other High-level ideas

• define custom exceptions. Instead of using the error_sev and error_inp functions where you print errors, define custom exceptions like InvalidCountryValueError. Throw it with your custom message inside
• since you are posting it on github, consider organizing the project properly - add requirements.txt with the list of dependencies, add more documentation, tests - see more at Open Sourcing a Python Project the Right Way
• on the related topic: currently, there is only one way to use your program. Consider someone who wants to use your library as an API - not going through the standard in inputs, but calling a function asking for currency rates. Thinking about your program this way may help you to re-design it a bit, apply "Extract Method" and other refactoring methods. Also, if you would try to add tests, you will quickly realize that there is no easy way to unittest the program - usually a red flag when designing clean and modular APIs

Performance notes

• I'd use a set to keep the supported list of currencies. Since you check the input currencies to be valid with in, this should have a positive impact on performance:

  CURRENCIES = {
      'USD', 'JPY', 'BGN', 'CZK', 'DKK', 'GBP', 'HUF', 'PLN', 'RON', 'SEK', 'CHF', 'NOK', 'HRK', 'RUB', 'TRY',
      'AUD', 'BRL', 'CAD', 'CNY', 'HKD', 'IDR', 'ILS', 'INR', 'KRW', 'MXN', 'MYR', 'NZD', 'PHP', 'SGD', 'THB',
      'ZAR', 'ISK'
  }