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I've recently read in "Dive into Python 3" that,

"The readlines() method now returns an iterator, so it is just as efficient as xreadlines() was in Python 2".

Can anyone tell me, How does a readlines() return a list or an iterator in Python 3?

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In Python 3, the readlines() method returns an iterable object, specifically a file object iterator, rather than directly returning a list as it did in Python 2. This change was made to improve efficiency and memory usage when working with large files.

When you call readlines() in Python 3, it returns an iterator that can be used to iterate over the lines of the file one by one. This iterator behaves similarly to a list, allowing you to access each line sequentially. However, unlike a list, the iterator doesn't load the entire file into memory all at once, which can be beneficial for memory-intensive operations.

You can still perform operations on the iterator object returned by readlines(), such as looping over the lines or using it in list comprehensions. If you need a list of all the lines, you can convert the iterator to a list explicitly by using the list() function, like list(file.readlines()).

Overall, in Python 3, readlines() provides the flexibility of an iterator, making it memory-efficient for working with large files, while still allowing you to access the lines in a manner similar to a list if needed.
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Check the below code:

Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32 bit (Intel)] on win32

Type "help", "copyright", "credits" or "license" for more information.

>>> f = open('/junk/so/foo.txt')

>>> type(f.readlines())

<class 'list'>

>>> help(f.readlines)

Help on built-in function readlines:

readlines(...)

    Return a list of lines from the stream.

    hint can be specified to control the number of lines read: no more

    lines will be read if the total size (in bytes/characters) of all

    lines so far exceeds hint.

>>>

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In Python 3, the behavior of the readlines() method has been modified compared to Python 2. Instead of directly returning a list, it now returns an iterable object known as a file object iterator. This change was made to enhance efficiency and optimize memory usage, particularly when dealing with large files.

Upon calling readlines() in Python 3, you receive an iterator that allows you to iterate over the lines of the file sequentially. Although the iterator resembles a list in terms of line access, it differs in that it doesn't load the entire file into memory all at once. This memory-efficient approach can be advantageous for operations involving substantial amounts of data.

You can perform various operations on the iterator object obtained from readlines(), including iterating over lines or utilizing it in list comprehensions. Should you require a list containing all the lines, you can explicitly convert the iterator to a list using the list() function, such as list(file.readlines()).

Overall, in Python 3, readlines() offers the flexibility of an iterator, prioritizing memory efficiency when handling sizable files, while still providing the capability to access lines in a manner similar to a list if desired.

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In Python 3, the readlines() method no longer returns a list directly as it did in Python 2. Instead, it now returns an iterable object called a file object iterator. This change was implemented to optimize efficiency and minimize memory usage, particularly when dealing with large files.

When you call readlines() in Python 3, you receive an iterator that allows you to iterate over the lines of the file one by one. Although similar to a list in terms of line access, the iterator does not load the entire file into memory at once. This adjustment ensures efficient memory usage during operations involving substantial data.

You can perform various operations on the iterator object obtained from readlines(), including iterating over lines or utilizing it in list comprehensions. To obtain a list containing all the lines, you can explicitly convert the iterator to a list using the list() function, as in list(file.readlines()).

Overall, in Python 3, the readlines() method provides the benefits of an iterator, prioritizing memory efficiency when working with large files, while still allowing convenient line access when necessary.

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