\u001b[0m in \u001b[0;36ma_func\u001b[0;34m()\u001b[0m\n\u001b[1;32m 2\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 3\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0ma_func\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 4\u001b[0;31m \u001b[0ma_var\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0ma_var\u001b[0m \u001b[0;34m+\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 5\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0ma_var\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'[ a_var inside a_func() ]'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 6\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;31mUnboundLocalError\u001b[0m: local variable 'a_var' referenced before assignment"
]
},
{
"output_type": "stream",
"stream": "stdout",
"text": [
"1 [ a_var outside a_func() ]\n"
]
}
],
"prompt_number": 4
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"
\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"
\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 2. LEG - Local, Enclosed, and Global scope\n",
"\n",
"\n",
"\n",
"Now, let us introduce the concept of the enclosed (E) scope. Following the order \"Local -> Enclosed -> Global\", can you guess what the following code will print?"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example 2.1**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a_var = 'global value'\n",
"\n",
"def outer():\n",
" a_var = 'enclosed value'\n",
" \n",
" def inner():\n",
" a_var = 'local value'\n",
" print(a_var)\n",
" \n",
" inner()\n",
"\n",
"outer()"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 4
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**a)**\n",
"global value
\n",
"\n",
"**b)** \n",
"enclosed value
\n",
"\n",
"**c)** \n",
"local value
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"[[go to solution](#solutions)]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### Here is why:\n",
"\n",
"Let us quickly recapitulate what we just did: We called `outer()`, which defined the variable `a_var` locally (next to an existing `a_var` in the global scope). Next, the `outer()` function called `inner()`, which in turn defined a variable with of name `a_var` as well. The `print()` function inside `inner()` searched in the local scope first (L->E) before it went up in the scope hierarchy, and therefore it printed the value that was assigned in the local scope."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Similar to the concept of the `global` keyword, which we have seen in the section above, we can use the keyword `nonlocal` inside the inner function to explicitely access a variable from the outer (enclosed) scope in order to modify its value. \n",
"Note that the `nonlocal` keyword was added in Python 3.x and is not implemented in Python 2.x (yet)."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a_var = 'global value'\n",
"\n",
"def outer():\n",
" a_var = 'local value'\n",
" print('outer before:', a_var)\n",
" def inner():\n",
" nonlocal a_var\n",
" a_var = 'inner value'\n",
" print('in inner():', a_var)\n",
" inner()\n",
" print(\"outer after:\", a_var)\n",
"outer()"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"outer before: local value\n",
"in inner(): inner value\n",
"outer after: inner value\n"
]
}
],
"prompt_number": 5
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"
\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 3. LEGB - Local, Enclosed, Global, Built-in\n",
"\n",
"To wrap up the LEGB rule, let us come to the built-in scope. Here, we will define our \"own\" length-funcion, which happens to bear the same name as the in-built `len()` function. What outcome do you excpect if we'd execute the following code?"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example 3**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a_var = 'global variable'\n",
"\n",
"def len(in_var):\n",
" print('called my len() function')\n",
" l = 0\n",
" for i in in_var:\n",
" l += 1\n",
" return l\n",
"\n",
"def a_func(in_var):\n",
" len_in_var = len(in_var)\n",
" print('Input variable is of length', len_in_var)\n",
"\n",
"a_func('Hello, World!')"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 6
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**a)**\n",
"raises an error (conflict with in-built `len()` function)
\n",
"\n",
"**b)** \n",
"called my len() function\n",
"Input variable is of length 13\n",
"\n",
"**c)** \n",
"Input variable is of length 13
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"[[go to solution](#solutions)]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### Here is why:\n",
"\n",
"Since the exact same names can be used to map names to different objects - as long as the names are in different name spaces - there is no problem of reusing the name `len` to define our own length function (this is just for demonstration pruposes, it is NOT recommended). As we go up in Python's L -> E -> G -> B hierarchy, the function `a_func()` finds `len()` already in the global scope first before it attempts"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"
\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Self-assessment exercise"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Now, after we went through a couple of exercises, let us quickly check where we are. So, one more time: What would the following code print out?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a = 'global'\n",
"\n",
"def outer():\n",
" \n",
" def len(in_var):\n",
" print('called my len() function: ', end=\"\")\n",
" l = 0\n",
" for i in in_var:\n",
" l += 1\n",
" return l\n",
" \n",
" a = 'local'\n",
" \n",
" def inner():\n",
" global len\n",
" nonlocal a\n",
" a += ' variable'\n",
" inner()\n",
" print('a is', a)\n",
" print(len(a))\n",
"\n",
"\n",
"outer()\n",
"\n",
"print(len(a))\n",
"print('a is', a)"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 59
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"[[go to solution](#solutions)]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Conclusion"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"I hope this short tutorial was helpful to understand the basic concept of Python's scope resolution order using the LEGB rule. I want to encourage you (as a little self-assessment exercise) to look at the code snippets again tomorrow and check if you can correctly predict all their outcomes."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### A rule of thumb"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"In practice, **it is usually a bad idea to modify global variables inside the function scope**, since it often be the cause of confusion and weird errors that are hard to debug. \n",
"If you want to modify a global variable via a function, it is recommended to pass it as an argument and reassign the return-value. \n",
"For example:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a_var = 2\n",
"\n",
"def a_func(some_var):\n",
" return 2**3\n",
"\n",
"a_var = a_func(a_var)\n",
"print(a_var)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"8\n"
]
}
],
"prompt_number": 42
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"
\n",
"
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Solutions\n",
"\n",
"In order to prevent you from unintentional spoilers, I have written the solutions in binary format. In order to display the character representation, you just need to execute the following lines of code:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('Example 1.1:', chr(int('01100011',2)))"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 6
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('Example 1.2:', chr(int('01100001',2)))"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 7
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('Example 2.1:', chr(int('01100011',2)))"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 8
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('Example 3.1:', chr(int('01100010',2)))"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 9
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"# Execute to run the self-assessment solution\n",
"\n",
"sol = \"000010100110111101110101011101000110010101110010001010\"\\\n",
"\"0000101001001110100000101000001010011000010010000001101001011100110\"\\\n",
"\"0100000011011000110111101100011011000010110110000100000011101100110\"\\\n",
"\"0001011100100110100101100001011000100110110001100101000010100110001\"\\\n",
"\"1011000010110110001101100011001010110010000100000011011010111100100\"\\\n",
"\"1000000110110001100101011011100010100000101001001000000110011001110\"\\\n",
"\"1010110111001100011011101000110100101101111011011100011101000100000\"\\\n",
"\"0011000100110100000010100000101001100111011011000110111101100010011\"\\\n",
"\"0000101101100001110100000101000001010001101100000101001100001001000\"\\\n",
"\"0001101001011100110010000001100111011011000110111101100010011000010\"\\\n",
"\"1101100\"\n",
"\n",
"sol_str =''.join(chr(int(sol[i:i+8], 2)) for i in range(0, len(sol), 8))\n",
"for line in sol_str.split('\\n'):\n",
" print(line)"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 58
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"
\n",
"
\n",
""
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Warning: For-loop variables \"leaking\" into the global namespace"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"In contrast to some other programming languages, `for-loops` will use the scope they exist in and leave their defined loop-variable behind.\n"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"for a in range(5):\n",
" if a == 4:\n",
" print(a, '-> a in for-loop')\n",
"print(a, '-> a in global')"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"4 -> a in for-loop\n",
"4 -> a in global\n"
]
}
],
"prompt_number": 5
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**This also applies if we explicitely defined the `for-loop` variable in the global namespace before!** In this case it will rebind the existing variable:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"b = 1\n",
"for b in range(5):\n",
" if b == 4:\n",
" print(b, '-> b in for-loop')\n",
"print(b, '-> b in global')"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"4 -> b in for-loop\n",
"4 -> b in global\n"
]
}
],
"prompt_number": 9
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"However, in **Python 3.x**, we can use closures to prevent the for-loop variable to cut into the global namespace. Here is an example (exectuted in Python 3.4):"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"i = 1\n",
"print([i for i in range(5)])\n",
"print(i, '-> i in global')"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"[0, 1, 2, 3, 4]\n",
"1 -> i in global\n"
]
}
],
"prompt_number": 1
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Why did I mention \"Python 3.x\"? Well, as it happens, the same code executed in Python 2.x would print:\n",
"\n",
"\n",
"4 -> i in global\n",
""
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"This goes back to a change that was made in Python 3.x and is described in [What\u2019s New In Python 3.0](https://docs.python.org/3/whatsnew/3.0.html) as follows:\n",
"\n",
"\"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.\""
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [],
"language": "python",
"metadata": {},
"outputs": []
}
],
"metadata": {}
}
]
}