What happens if names = {} is placed inside the loop?

[Rajeev Bagra]

Here is a forum-ready, polished explanation you can directly publish:

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What happens if names = {} is placed inside the loop?

Original (Correct) Code

def load_file(self):
names = {}
with open(self.__filename) as f:
for line in f:
parts = line.strip().split(';')
name, *numbers = parts
names[name] = numbers
return names

In this version, the dictionary names is created once, before the loop begins.
Each line from the file adds new data to the same dictionary, so all records are preserved.

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If names = {} is moved inside the loop

def load_file(self):
with open(self.__filename) as f:
for line in f:
names = {} # Reset happens here
parts = line.strip().split(';')
name, *numbers = parts
names[name] = numbers
return names

What changes?

Placing names = {} inside the loop causes the dictionary to be recreated on every iteration.
This means any previously stored data is wiped out each time a new line is read.

As a result, the function will only return data from the last line of the file.

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Practical Example

File contents:

John;123;456
Mary;789;111
Alex;222;333

Output when names is inside the loop:

{'Alex': ['222', '333']}

Only the final line survives.

Output when names is outside the loop:

{
'John': ['123', '456'],
'Mary': ['789', '111'],
'Alex': ['222', '333']
}

All lines are correctly stored.

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Why this happens

Each loop cycle does this:

1. Creates a new empty dictionary
2. Stores one line’s data
3. Discards the dictionary in the next iteration

So the previous data never accumulates.

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Key Takeaway

✅ Create data containers outside loops when accumulating results
❌ Creating them inside loops resets them every iteration

Rule of thumb:

If you want to collect data across multiple lines, initialize the container before the loop.

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This distinction is crucial when working with file processing, logs, or any form of data aggregation in Python.

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Q&A: Where Should names = {} Be Placed in This Python File-Reading Loop?

Question:

I have the following Python class:

class FileHandler:
    def __init__(self, filename):
        self.__filename = filename

    def load_file(self):
        with open(self.__filename) as f:
            names = {}    # ← What if I place it here?
            for line in f:
                parts = line.strip().split(';')
                name, *numbers = parts
                names[name] = numbers

        return names

Usually, names = {} is placed before the with block.
My question is:

If I place names = {} inside the with open(...) block but before the loop, will the behavior change? Or is it exactly the same?

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Answer:

Short answer:

Yes — it still works correctly.
No — the behavior does not change at all.

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Detailed Explanation

The important detail is when the dictionary is created.

✔️ When names = {} is placed before the loop

It is created only once, before any lines are processed.
Every line in the file adds new data to the same dictionary.

✔️ When names = {} is placed inside the with block but still before the loop

Example:

with open(self.__filename) as f:
    names = {}      # Created once here
    for line in f:
        ...

This is still outside the loop, so the dictionary is created only one time.
The result is the same: all lines are added to the same dictionary, nothing is lost.

❌ The only incorrect placement is inside the loop

for line in f:
    names = {}   # WRONG: resets on every iteration

This recreates the dictionary on each line, wiping out previous data.
You would end up with only the last line in your output.

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Final Comparison

Placement of names = {}	Created Once?	Correct?	Result
Before with open()	Yes	✔️	All lines stored
Inside with, before loop	Yes	✔️	All lines stored
Inside the loop	No	❌	Only last line stored

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Conclusion

• Placing names = {} before the loop (whether inside or outside with) works perfectly.
• It does not change the behavior of the code.
• The code only breaks if you place names = {} inside the loop, because it resets each iteration.

This behavior is the same for any accumulator: lists, dicts, counters, etc.

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