Python Basics for Data Science

Question 1

What can be used as a shortcut for a function with return? Give an example with 2 variables.

Answer 1

lambda. Example: lambda x,y: x+9-y
x,y= 3,2 #10
Note:
- lambda function doesn´t have a return statement

Question 2

What are exceptions, why and how are they helpful?

Answer 2

try/except.
prevent the origram from crashing
difficult part is placed into try statement, alternative in except

Question 3

How do you check the existance of an element in a list?

Answer 3

4 in [3,6,7] #gives booleons back

also possible when list is a variable like a. 2 in a …

Question 4

How do you unpack a list of 3 digits?

How do you proceed if you only want two of the digits?

Answer 4

x,y,z=[8,9,1]. Now x=8 …

_,y,z=[8,9,1]: underscore marks what gets out

Question 5

How do you swap 2 variables?

Answer 5

y,x=5,6

y,x=x,y # now x=6 and y=5

Question 6

You want to get a pair out of a dic?

Answer 6

dicname.get(“key”) # value is optional

returns None when no key is found(other methods produce errors)

Question 7

How to add a new entry or replace in a dic?

Answer 7

dicname[“keyname”]= value

Question 8

What to do if you want to see the keys, values and key-value tuples of a dic?

Answer 8

dicname.keys()/.vales()/.items()

Question 9

Counting word appearances in a text with a dic. How? Code it.

Also name a shortcut if you look up a key, it´s not in, so it directly creates one. How would you code the same example?

Give an even shorter shortcut and the code.
Then, tell me how do I get the 10 most occuring values of that dic? How do you code it?

Answer 9

word_count={}
for word in text:
    if word in word_count:
        word_count["word"] +=1
    else:
         word_count["word"] =1

Shortcut= defaultdict

word_count=defaultdict(int) # int creates a 0
for word in text:
word_counts[“word”] +=1

shorter shortcut= counter

from collections import counter

word_counts=Counter(document)

for word, count in word_counts.most_common(10)
print word, count

Question 10

How to test a condition faster than with if-else? Give a code example.

Answer 10

Ternary Operators

good_weather=true
forecast= “sun” if good_weather else “rain”

Question 11

sorted(x) vs x.sort(). Tell me the similarities and differences.

Answer 11

x.sort() only sorts list, other sorts everythin iterable, p.e. also dics.

sorted(x) is used to make a new list with variable y and not overwrite it like x.sort()

Both take additional arguments. y=(sorted, dic, key=lambda actors: actors[2], reverse=) #key now sorts by points and not age. A key changes a variable before its get compared and listed

Question 12

What is a generator good for? Give a code example. What has yield to do with that?

Answer 12

loops go often over iterable objects and therefore the memory shrinkens. To prevent this, we use a generator, who goes over it once. Only the generator gets back to you.

1st example:

def firstn(n):
   3     num = 0
   4     while num < n:
   5         yield num
   6         num += 1

yield works like the return statement

2nd example (with list comprehension)

doubles = [2 * n for n in range(50)]
 # same as the list comprehension above
 doubles = list(2 * n for n in range(50))

Question 13

What does …

1. random.random()
2. random.randrange()
3. random.shuffle(listname)
4. random.choice([…,…,…])
5. random.sample(listname, number of wanted elements)

Answer 13

1. produces random number between 0-1
2. takes random number of given range
3. reorders list
4. picks random element out of list
5. easy

Question 14

What is functools.partial. Code an example!

Answer 14

Because def gets treated like an object in Python, f.p. calls another definition, with providing 1 argument where 2 are needed.

def sum(a,b)
       return a+b
incr=functools.partial(sum,1)
incr(3)          
#ergibt 4         

# incr behaves like sum, yet misses one argument. Thus it takes the one from functools.partial.

Question 15

Code an alternative to list comprehension with the map() function. # double the numbers of a list with a self created function.

Answer 15

ys=map(double,xs)

Question 16

What is enumerate helpfull for? Code an example. Then code an example if you only want the indices.

Answer 16

with enumerate you loop over sth and automatically count.

for counter, value in enumerate(listname)
print(counter, value)

and

# in this example index is counter
for i,_ in enumerate(listname) 
     do_sth_with(i)       #p.e. print()

Question 17

What can you do with zip()? And how to unzip?

Answer 17

putting lists together.
xs=[a,b,c]
ys=[1,2,3]
tip(xs,ys)
= [("a",1),...]

unzip:

zip((“a”,1),…)

Question 18

What do args do? Give a code example!

What does kwargs do? Give an example!

Answer 18

With args you can out as many arguments as you wish into a function, which is helpful if the number of arguments is defined in the process.

Code example:

def sum(*args)  # asteriks important 
result=0
for x in args
result+=X
return result
print(sum(34,6,7,3,2))   # give a tuple instead of a list

kwargs maps keywords with arguments.
def: my_fun(**kwargs)
for key, value in kwargs.items():

my_fun(first="I", mid="am", last="good")
print
first==I
mid==am
last==good