Hello, and welcome back! Are you ready for a new challenge? In this unit, we're stepping up a notch to tackle a complex yet intriguing task. It involves parsing complex strings into Python dictionaries and then updating them, a common requirement in many real-world tasks. So yes, this unit's session is going to be pretty pragmatic — just the way you like it!

This task involves transforming a given string into a nested dictionary and updating a specific key-value pair within that dictionary. The input string will take the form "Key1=Value1,Key2=Value2,...". When a part of the value is another key-value string, we create a nested dictionary.

For example, the string "A1=B1,C1={D1=E1,F1=G1},I1=J1" should be transformed into the following nested dictionary:

Python
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1{
2'A1': 'B1',
3'C1': {
4 'D1': 'E1',
5 'F1': 'G1'
6 },
7'I1': 'J1'
8}

Your Python function should parse this string into the above dictionary, then update the value of the nested key F1 from G1 to some other value, say 'NewValue'. The function should ultimately return the updated dictionary.

Our first step here will be to parse the string into a nested dictionary. Before diving into coding, let us understand that simple string splitting methods do not handle hierarchical structures well, hence, our solution needs to account for the nested nature of our text.

As you parse the string, you'll have to read through each character and note its role. Here's a hint: every time the '=' symbol is encountered, you can infer that the text bit you have on your hands is a key. Further, upon hitting the ',' symbol, you should gather that you've just finished reading a value. At this stage, it is safe to add the key-value pair you've just read into the dictionary you're constructing.

The next step calls for special attention to the braces. When you encounter an opening brace '{', you should know you are taking a step deeper into the hierarchy. On the other hand, when you encounter a closing brace '}', you are taking a step back up.

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1def parse_string(input_string):
2    # Preprocess the input_string: replace commas outside of curly brackets
3    preprocessed_string = ""
4    balance = 0
5    for char in input_string:
6        if char == "{":
7            balance += 1
8        elif char == "}":
9            balance -= 1
10        elif char == "," and balance == 0:
11            preprocessed_string += ";"
12            continue
13        preprocessed_string += char
14            
15    # Parse to a nested dictionary
16    elements = preprocessed_string.split(";")
17    result_dict = {}
18    for element in elements:
19        key, value = element.split("=", 1)
20        if "{" in value:
21            # Value is a nested dictionary
22            nested_dict = parse_string(value[1:-1])
23            result_dict[key] = nested_dict
24        else:
25            result_dict[key] = value
26    return result_dict

Now we have the parsed dictionary. We can move into the final phase of the task: updating a specific key-value pair. We will scour the dictionary for the specified key, which might be found at the base level or nested within another key's value. If it's a nested key-value pair, we would be updating the value within the nested dictionary. If it's a base level key-value pair, the update will be in the base dictionary.

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1def update_dict(dictionary, key, value):
2    # Iterate through all the key-value pairs in the dictionary
3    for k in dictionary:
4        # If the key matches the required key, update its value
5        if k == key:
6            dictionary[k] = value
7        # If the value is a nested dictionary, recursively search for the key inside it
8        elif isinstance(dictionary[k], dict):
9            update_dict(dictionary[k], key, value)
10            
11def parse_string_and_update_value(input_string, update_key, new_value):
12    # Parse the given string into a dictionary
13    dictionary = parse_string(input_string)
14    # Update the value of the specified key in the dictionary
15    update_dict(dictionary, update_key, new_value)
16    # Return the updated dictionary
17    return dictionary

The final parse_string_and_update_value function pulls together all pieces of our solution. First, it parses the string, then it updates the requested key-value pair.

Well done! You've completed an intensive hands-on session dealing with complex strings and nested dictionaries in Python. This type of task often mirrors real-life scenarios where you process complex data and make updates based on particular criteria.

Now it's your turn to reinforce what you've learned in this unit. Try practicing with different strings and attempting to update various key-value pairs. I'm confident that with practice, you'll be able to apply these coding strategies to a wide range of problems. Until our next learning adventure, happy coding!