Step 2 - CLI Arguments

In the previous step, we were able to prompt a user for input. This is useful for human input, but more often in AWS we want to automate our workflow, which means we need a method of passing parameters via the Command Line Interface (CLI), for example when using a script

Create a new file called lab_5_step_2_cli_arguments.py and add the following code:

import boto3

def translate_text(**kwargs): 
    client = boto3.client('translate')
    response = client.translate_text(**kwargs)
    print(response) 

### Change below this line only ###

text = input("Provide the text you want translating: ")
source_language_code = input("Provide the two letter source language code: ")
target_language_code = input("Provide the two letter target language code: ") 

def main():
    translate_text(
        Text=text, 
        SourceLanguageCode=source_language_code,
        TargetLanguageCode=target_language_code
        )

if __name__=="__main__":
    main()

Using argparse to input arguments from the CLI

Python has a built-in argument parser called argparse. It makes it easy to write user-friendly command-line interfaces. There is a lot in the next section, but don’t worry, it should be simple to figure out.

Modify your program to include the code below:

import argparse # argparse is a built in python package, we add it with an import statement
import boto3

# Define the parser variable to equal argparse.ArgumentParser()
parser = argparse.ArgumentParser(description="Provides translation between one source language and another of the same set of languages.")

# Add each of the arguments using the parser.add_argument() method
parser.add_argument(
    '--text',
    dest="Text",
    type=str,
    help="The text to translate. The text string can be a maximum of 5,000 bytes long. Depending on your character set, this may be fewer than 5,000 characters",
    required=True
    )

parser.add_argument(
    '--source-language-code', 
    dest="SourceLanguageCode", 
    type=str, 
    help="The language code for the language of the source text. The language must be a language supported by Amazon Translate.",
    required=True
    )

parser.add_argument(
    '--target-language-code',
    dest="TargetLanguageCode",
    type=str,
    help="The language code requested for the language of the target text. The language must be a language support by Amazon Translate.",
    required=True
    )

# This will inspect the command line, convert each argument to the appropriate type and then invoke the appropriate action.
args = parser.parse_args()

def translate_text(**kwargs): 
    client = boto3.client('translate')
    response = client.translate_text(**kwargs)
    print(response) 

def main():
    # vars() is an inbuilt function which returns a dictionary object
    translate_text(**vars(args))

if __name__=="__main__":
    main()

  • To run the program, enter the following command in the terminal:

    python lab_5_step_2_cli_arguments.py --text "we are learning python on AWS" --source-language-code en --target-language-code fr

  • We have added the parameters on the command line.

  • Each parameter relates to each block for parser.add_argument

There is a lot of new code in this example so we will break it down below.

What did we do?

  • We imported a python package called argparse using import argparse

  • We created an ArgumentParser object which holds the information necessary to parse the command line into python data types

  • We used parser.add_argument() to add each of the arguments. This has the following syntax

    • Argument name for example --text
    • The dest is required to make it a value keyword argument accepted by the parameter, otherwise it will use the default argument text="value" which should be Text="value"
    • type is the data type which in this case is a string (str)
    • help is the help text shown

  • args = parser.parse_args() will inspect the command line, convert each argument to the appropriate type and then invoke the appropriate action

  • Now when we call the translate_text() function we need to use **vars(args). The **vars turns our object created by args = parser.parse_args() into a dictionary object which we can pass as key:value pairs to our function

What did python do?

  • Python used the arguments we provided as key:value pairs. It passed them to the translate_text() function.

Exercises

If you have time try the following:

  • The translate_text() function of boto3 has a 4th parameter called TerminologyNames. Add this as an optional argument.
  • Print out the value of args using print(args) What does it say?
  • Remove dest from parser.add_argument() and look at the values returned by print(args). Why does this now raise an error and what is dest doing?
  • Argparse has a lot more features, check out the documentation
  • In the terminal, use the aws cli command aws translate translate-text help. Compare your code to what is provided in the cli help documentation for the Amazon Translate service.