Python Basics for Data Science Flashcards
What can be used as a shortcut for a function with return? Give an example with 2 variables.
lambda. Example: lambda x,y: x+9-y
x,y= 3,2 #10
Note:
- lambda function doesn´t have a return statement
What are exceptions, why and how are they helpful?
try/except.
prevent the origram from crashing
difficult part is placed into try statement, alternative in except
How do you check the existance of an element in a list?
4 in [3,6,7] #gives booleons back
also possible when list is a variable like a. 2 in a …
How do you unpack a list of 3 digits?
How do you proceed if you only want two of the digits?
x,y,z=[8,9,1]. Now x=8 …
_,y,z=[8,9,1]: underscore marks what gets out
How do you swap 2 variables?
y,x=5,6
y,x=x,y # now x=6 and y=5
You want to get a pair out of a dic?
dicname.get(“key”) # value is optional
returns None when no key is found(other methods produce errors)
How to add a new entry or replace in a dic?
dicname[“keyname”]= value
What to do if you want to see the keys, values and key-value tuples of a dic?
dicname.keys()/.vales()/.items()
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?
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
How to test a condition faster than with if-else? Give a code example.
Ternary Operators
good_weather=true
forecast= “sun” if good_weather else “rain”
sorted(x) vs x.sort(). Tell me the similarities and differences.
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
What is a generator good for? Give a code example. What has yield to do with that?
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))
What does …
- random.random()
- random.randrange()
- random.shuffle(listname)
- random.choice([…,…,…])
- random.sample(listname, number of wanted elements)
- produces random number between 0-1
- takes random number of given range
- reorders list
- picks random element out of list
- easy
What is functools.partial. Code an example!
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.
Code an alternative to list comprehension with the map() function. # double the numbers of a list with a self created function.
ys=map(double,xs)
What is enumerate helpfull for? Code an example. Then code an example if you only want the indices.
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()
What can you do with zip()? And how to unzip?
putting lists together.
xs=[a,b,c]
ys=[1,2,3]
tip(xs,ys)
= [("a",1),...]
unzip:
zip((“a”,1),…)
What do args do? Give a code example!
What does kwargs do? Give an example!
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
