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Add example - Leaking variables to global scope
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wtfpy.md
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wtfpy.md
@ -204,6 +204,69 @@ for x in range(7):
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[0, 1, 2, 3, 4, 5, 6]
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```
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## Loop variables leaking out of local scope!
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1.
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```py
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for x in range(7):
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if x == 6:
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print(x, ': for x inside loop')
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print(x, ': x in global')
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```
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**Output:**
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```py
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6 : for x inside loop
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6 : x in global
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```
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But `x` was never defined ourtside the scope of for loop...
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2.
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```py
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# This time let's initialize x first
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x = -1
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for x in range(7):
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if x == 6:
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print(x, ': for x inside loop')
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print(x, ': x in global')
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```
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**Output:**
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```py
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6 : for x inside loop
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6 : x in global
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```
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3.
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```
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x = 1
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print([x for x in range(5)])
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print(x, ': x in global')
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```
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**Output (on Python 2.x):**
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```
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[0, 1, 2, 3, 4]
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(4, ': x in global')
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```
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**Output (on Python 3.x):**
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```
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[0, 1, 2, 3, 4]
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1 : x in global
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```
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### Explanation
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In Python for-loops use the scope they exist in and leave their defined loop-variable behind. This also applies if we explicitly defined the for-loop variable in the global namespace before. In this case it will rebind the existing variable.
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The differences in the output of Python 2.x and Python 3.x interpreters for list comprehension example can be explained by following change documented in [What’s New In Python 3.0](https://docs.python.org/3/whatsnew/3.0.html) documentation:
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> "List comprehensions no longer support the syntactic form `[... for var in item1, item2, ...]`. Use `[... for var in (item1, item2, ...)]` instead. Also note that list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a `list()` constructor, and in particular the loop control variables are no longer leaked into the surrounding scope."
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## A tic-tac-toe where X wins in first attempt!
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```py
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@ -316,7 +379,29 @@ Quoting from https://docs.python.org/2/reference/datamodel.html
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> Immutable sequences
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An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)
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##
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## Using a varibale not defined in scope
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```py
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a = 1
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def some_func():
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return a
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def another_func():
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a += 1
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return a
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```
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**Output:**
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```py
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>>> some_func()
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1
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>>> another_func()
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UnboundLocalError: local variable 'a' referenced before assignment
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```
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**Explanation:**
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When you make an assignment to a variable in a scope, it becomes local to that scope. So `a` becomes local to the scope of `another_func` but it has not been initialized previously in the same scope which throws an error. Read [this](http://sebastianraschka.com/Articles/2014_python_scope_and_namespaces.html) short but awesome guide to learn more about how namespaces and scope resolution works in Python.
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# Contributing
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