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Add more examples and change description slightly.
* In place update of mutables. * Modifying a list while iterating.
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README.md
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README.md
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> A collection of tricky Python examples
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Python being an awesome higher level language, provides us many functionalities for our comfort. But sometimes, the outcomes may not seem obvious to a normal Python user at the first sight.
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Python being an awesome higher level language, provides us many functionalities for programmer's comfort. But sometimes, the outcomes may not seem obvious to a normal Python user at the first sight.
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Here's an attempt to collect such classic and tricky examples of unexpected behaviors in Python and see what exactly is happening under the hood! I find it a nice way to learn internals of a language and I think you'll like them as well!
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Here's an attempt to collect such classic and tricky examples of unexpected behaviors in Python and see what exactly is happening under the hood! Anyways, I find it a nice way to learn internals of a language and I think you'll like them as well!
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- If you're an beginner to intermdediate level Python programmer, I'd personally recommend you to go through all of the examples below, as being aware about such pitfalls may be able to save a lot of debugging time in your future.
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- If you're an experienced Python programmer, you might be familiar with most of these examples, and I might be able to bring up some nice old memories of being bitten by the gotchas.
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So, here ya go...
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# Table of Contents
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@ -61,6 +66,8 @@ Here's an attempt to collect such classic and tricky examples of unexpected beha
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# 👀 Examples
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**Environment:** All the examples mentioned below are run on Python 3.5.2 interactive interpreter unless explicitly specified.
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## Example heading
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One line of what's happening:
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@ -771,6 +778,86 @@ class SomeClass:
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- A generator expression has its own scope.
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- Starting in 3.X, list comprehensions also have their own scope.
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## In-place update functions of mutable object types
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```py
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some_list = [1, 2, 3]
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some_dict = {
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"key_1": 1,
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"key_2": 2,
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"key_3": 3
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}
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some_list = some_list.append(4)
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some_dict = some_dict.update({"key_4": 4})
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```
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**Output:**
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```py
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>>> print(some_list)
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None
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>>> print(some_dict)
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None
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```
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### Explaination
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The functions like append
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## Deleting a list item while iterating over it
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```py
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list_1 = [1, 2, 3, 4]
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list_2 = [1, 2, 3, 4]
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list_3 = [1, 2, 3, 4]
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list_4 = [1, 2, 3, 4]
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for idx, item in enumerate(list_1):
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del item
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for idx, item in enumerate(list_2):
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list_2.remove(item)
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for idx, item in enumerate(list_3[:]):
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list_3.remove(item)
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for idx, item in enumerate(list_4):
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list_4.pop(idx)
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```
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**Output:**
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```py
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>>> list_1
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[1, 2, 3, 4]
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>>> list_2
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[2, 4]
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>>> list_3
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[]
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>>> list_4
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[2, 4]
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```
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### Explanation
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* Python does not support modifying a `list` while iterating over it. The correct way to do so is to iterate over a copy of the `list` instead, and `list_3[:]` does just that.
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```py
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>>> some_list = [1, 2, 3, 4]
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>>> id(some_list)
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139798789457608
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>>> id(some_list[:]) # Notice that python creates new object for sliced list.
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139798779601192
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```
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**Difference between `del`, `remove`, and `pop`:**
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* `remove` removes the first matching value, not a specific index, raises `ValueError` if value is not found.
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* `del` removes a specific index (That's why first `list_1` was unaffected), raises `IndexError` if invalid index is specified.
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* `pop` removes element at specific index and returns it, raises `IndexError` if invalid index is specified.
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* **Why the output is `[2, 4]`?** The list iteration is done index by index, and when we remove `1` from `list_2` or `list_4`, the contents of the lists are now `[2, 3, 4]`. Now `2` is at index 0 and `3` is at index 1. Since the next iteration is going to look at index 1 (which is the `3`), the `2` gets skipped entirely. Similar thing will happen with every alternate element in the list sequence.
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## Minor ones
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- `join()` is a string operation instead of list operation. (sort of counter-intuitive at first usage)
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@ -839,6 +926,7 @@ The idea and design for this list is inspired from Denys Dovhan's awesome projec
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* https://www.youtube.com/watch?v=sH4XF6pKKmk
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* https://www.reddit.com/r/Python/comments/3cu6ej/what_are_some_wtf_things_about_python
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* https://sopython.com/wiki/Common_Gotchas_In_Python
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* https://stackoverflow.com/questions/530530/python-2-x-gotchas-and-landmines
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# 🎓 License
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