Dive_into_Functions.rst

Dive into Functions

Exercises

8   Exercise

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

Hint locals print a dictionary with local variable as keys and their respective values.

x = 4

def func():
   y  = 5
   print locals()

>>> func()
{'y': 5}
>>> print x
4

9   Exercise

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

Hint locals print a dictionary with local variable as keys and their respective values.

x = 4

def func():
   y = 5
   x = 8
   print locals()
   x = x + 2

>>> y = func()
{'y': 5, 'x': 8}
>>>
>>> print y
None
>>> print x
4

10   Exercise

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

Hint locals print a dictionary with local variable as keys and their respective values.

x = 4

def func(a):
   y = x + 2
   print locals()
   x = y
   return y

>>> y = func(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in func
UnboundLocalError: local variable 'x' referenced before assignment

Unlike what we might think in y = x + 2 x is not get from the global scope. As soon as you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope. even if the assignment appear later in code. Here x = y make x as local variable whatever you are in func. so at line y = x + 2 we try to use the local variable x but we have to asign it a value (it is done later) so Python raise an UnboundLocalError (see python faq for details)

11   Exercise

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

Hint locals print a dictionary with local variable as keys and their respective values.

x = 4

def func(a):
   x = x + 2
   print locals()
   return x

y = func(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in func
UnboundLocalError: local variable 'x' referenced before assignment

print y
print y == x

12   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = 4

def func(x):
   x = x + 2
   return x

y = func(x)

>>> print x
4
>>> print y == x
False

13   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

def func():
   y = x
   return y

>>> x = 4
>>> z = func()
>>>
>>> print x
4
>>> print z
4
>>> print id(z) == id(x)
True

14   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = 4

def func(x = 5):
   x = x + 2
   return x

>>> y = func(x)
>>>
>>> print x
4
>>> print y
6

15   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

Hint locals print a dictionary with local variable as keys and their respective values.

x = 4

def func(a):
   global x
   def func2():
      print locals()
      y = x + 4
      return y
   z = func2()
   return z

y = func(x)
{}
>>> print x
4
>>> print y
8

16   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = {'a' : 4}

def func(a):
   a['b'] = 5
   return a

y = func(x)

>>> print x
{'a': 4, 'b': 5}
>>> print y
{'a': 4, 'b': 5}
>>> print x is y
True

17   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = {'a' : 4}

def func(a):
   a['b'] = 5

y = func(x)

>>> print x
{'a': 4, 'b': 5}
>>> print y
None

18   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = {'a' : 4}

def func(a):
   x['b'] = 5
   def func2():
      a['b'] = 6
   return a

y = func(x)

print x
{'a': 4, 'b': 5}
print y
{'a': 4, 'b': 5}

a refer to same object as x

in func x does not exist, so x refer to global variable x

we mutate x

func2 is never executed

we return a so the same object referred by x

so y refer also the same object as x

19   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = {'a' : 4}

def func(a):
   x['b'] = 5
   def func2():
      a['b'] = 6
   func2()
   return a

y = func(x)

print x
{'a': 4, 'b': 6}

in this code func2 is executed

a is not in func2 namespace so python find it in enclosing namespace func

a refer the same object as x

20   Exercice

Without executing the code in Python interpreter, can you determine what the code below print out. help you by drawing diagram.

x = {'a' : 4}

def func(a):
   x['b'] = 5
   def func2(x):
      x['b'] = 6
   func2(a.copy())
   return a

y = func(x)

print x
{'a': 4, 'b': 5}

in this code x a variable x is defined locally in func2 and hide the global variable x

this local x refer to a shallow copy of the func local variable a which refer to the same object as global variable x

so func2 does not do a side effect on dictionary referred by x as in previous example.

21   Exercice

Use the code of the exercise 4.5.7 on the kmer. Make a function which compute all kmer of a given length in a sequence.

:download:`kmer.py <_static/code/kmer.py>` .

22   Exercise

Write a function translate that have a nucleic sequence as parameter, and return the translate sequence. We give you a genetic code :

code = {  'ttt': 'F', 'tct': 'S', 'tat': 'Y', 'tgt': 'C',
           'ttc': 'F', 'tcc': 'S', 'tac': 'Y', 'tgc': 'C',
           'tta': 'L', 'tca': 'S', 'taa': '*', 'tga': '*',
           'ttg': 'L', 'tcg': 'S', 'tag': '*', 'tgg': 'W',
           'ctt': 'L', 'cct': 'P', 'cat': 'H', 'cgt': 'R',
           'ctc': 'L', 'ccc': 'P', 'cac': 'H', 'cgc': 'R',
           'cta': 'L', 'cca': 'P', 'caa': 'Q', 'cga': 'R',
           'ctg': 'L', 'ccg': 'P', 'cag': 'Q', 'cgg': 'R',
           'att': 'I', 'act': 'T', 'aat': 'N', 'agt': 'S',
           'atc': 'I', 'acc': 'T', 'aac': 'N', 'agc': 'S',
           'ata': 'I', 'aca': 'T', 'aaa': 'K', 'aga': 'R',
           'atg': 'M', 'acg': 'T', 'aag': 'K', 'agg': 'R',
           'gtt': 'V', 'gct': 'A', 'gat': 'D', 'ggt': 'G',
           'gtc': 'V', 'gcc': 'A', 'gac': 'D', 'ggc': 'G',
           'gta': 'V', 'gca': 'A', 'gaa': 'E', 'gga': 'G',
           'gtg': 'V', 'gcg': 'A', 'gag': 'E', 'ggg': 'G'
      }

22.1   bonus

This function have to take the phase as parameter

:download:`translate.py <_static/code/translate.py>` .

23   Exercise

Write a program that calculates the similarity of 2 RNA sequences.

• To compute the similarity you need to parse a file containing the :download:`similarity matrix <_static/data/similarity_matrix>`.

  Hint: use the module containing the functions that handle a matrix from previous chapter. put this matrix.py file in a directory named "my_python_lib" in your home or Desktop and import it in your current program (the similarity script must be placed elsewhere).

• The similarity of the 2 sequences is the sum of base similarities. so you have to compare the first base of two sequences and use the matrix to get the similarity from the similarity table, on so on for all bases then sum these similarities.

First implementation

:download:`matrix.py <_static/code/matrix.py>` .

Second implementation

:download:`matrix2.py <_static/code/matrix2.py>` .