Python: Get the Indexes of Elements from a List (Counting Same Occurrences)

This is the "translation" of earlier JavaScript post about Counting Same Occurrences.

Here, I have Python 2.7.7.

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The concept
The main idea of this function is the same as the JavaScript version (link above - so that I don't have to re-explain it^{[I'm quite lazy that way]}).

In this snippet, I typed a function that generates a dictionary (in JavaScript: object) output from a list (in JavaScript: array) input.

The output consists of

list element: {"index": [array of indexes], "length": array of indexes length}

pairs.

The "confusing" part was when I tried to initialize the list type for the non-existing dictionary key.

The "solution" is using try-except block, with KeyError exception.

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Let's take a look
def occurrence(a): result = {} if isinstance(a, list): # Check input for i, v in enumerate(a): if not isinstance(v, int) or not isinstance(v, str): v = str(v) try: result[v].append(i) except KeyError: # List type and value initialization result[v] = [i] for i in result: result[i] = {"index": result[i], "length": len(result[i])} return result

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Result
I'm using pprint (Pretty Print) module here, so the output can be read conveniently.

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Accessing the output
To print the list of indexes, do this:

print occurrence(your_list)["the_element"]["index"]

And to print the list length (of a particular element):

print occurrence(your_list)["the_element"]["length"]

You can always store it as a variable and do further process.

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List of keywords

1. def
   A (custom) function declaration in Python: more info
2. isinstance(the_object, class_info)
   It is a built-in function to test whether the_object is an instance of class_info argument. Like instanceof operator in JavaScript (also in PHP and Java, Ruby has some methods you can choose).
   The the_object in the snippet above is denoted by v (an item from a list) and class_info is list: more info
3. for index, value in enumerate(iterable_input)
   A looping block which iterates over the iterable_input. With this modification, we can also access the index of a particular value. Like forEach in JavaScript or foreach in PHP (and Java has the funkiest form for that):
   • More info about for
   • More info about enumerate()
   • Q&A at Stack Overflow
4. if statement
   Just like JavaScript or any other programming languages conditional statement block: more info
5. not operator
   Used in Boolean operation. In JavaScript, it is an exclamation mark (!). It negates an argument: more info
6. str(the_input) function
   Convert the_input into string object. In JavaScript, it's like toString(): more info
7. append(an_item) method
   A method to append an_item into a list (which is declared in front of the method). In JavaScript, it is equivalent to push(): more info
8. try-except block
   It is like try-catch-[finally] error block handling in JavaScript:
   • More info about try-except
   • More info about list of built-in exceptions in Python
9. Other than those...
   are custom variable/parameter keywords.
   They are:
   • occurrence (the name of the function).
   • result (output).
   • v (item value).
   • i (index).
   • a (received list input).

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Using pprint - data pretty printer
Read more on pprint documentation