Why dict get key instead of dict key

Question

Today  I came across the dict method get which  given a key in the dictionary  returns the associated value   For what purpose is this function useful  If I wanted to find a value associated with a key in a dictionary  I can just do dict key   and it returns the same thing   dictionary     Name    Harry    Age   17  dictionary  Name   dictionary get  Name

User · Answer

For what purpose is this function useful?

One particular usage is counting with a dictionary. Let's assume you want to count the number of occurrences of each element in a given list. The common way to do so is to make a dictionary where keys are elements and values are the number of occurrences.

fruits = ['apple', 'banana', 'peach', 'apple', 'pear']
d = {}
for fruit in fruits:
    if fruit not in d:
        d[fruit] = 0
    d[fruit] += 1

Using the .get() method, you can make this code more compact and clear:

for fruit in fruits:
    d[fruit] = d.get(fruit, 0) + 1

User · Answer

Why dict get key  instead of dict key     0  Summary  Comparing to dict key   dict get provides a fallback value when looking up for a key   1  Definition  get key   default     4  Built-in Types     Python 3 6 4rc1 documentation      Return the value for key if key is in the dictionary  else default  If default is not given  it defaults to None  so that this method never raises a KeyError   d     Name    Harry    Age   17  In  4   d  gender   KeyError   gender  In  5   d get  gender    Not specified  please add it   Out 5    Not specified  please add it    2  Problem it solves   If without default value  you have to write cumbersome codes to handle such an exception   def get harry info key       try          return      format d key       except KeyError          return  Not specified  please add it  In  9   get harry info  Name   Out 9    Harry  In  10   get harry info  Gender   Out 10    Not specified  please add it    As a convenient solution  dict get introduces an optional default value avoiding above unwiedly codes   3  Conclusion  dict get has an additional default value option to deal with exception if key is absent from the dictionary

User · Answer

Based on usage should use this get method   Example1  In  14   user dict     type   False   In  15   user dict get  type        Out 15   False  In  16   user dict get  type   or     Out 16        Example2  In  17   user dict     type    lead    In  18   user dict get  type   or     Out 18    lead   In  19   user dict get  type        Out 19    lead

User · Answer

One difference  that can be an advantage  is that if we are looking for a key that doesn t exist we will get None  not like when we use the brackets notation  in which case we will get an error thrown   print dictionary get  address      None print dictionary  address      throws KeyError   address    Last thing that is cool about the get method  is that it receives an additional optional argument for a default value  that is if we tried to get the score value of a student  but the student doesn t have a score key we can get a 0 instead   So instead of doing this  or something similar    score   None try      score   dictionary  score   except KeyError      score   0   We can do this   score   dictionary get  score   0    score   0

User · Answer

I will give a practical example in scraping web data using python  a lot of the times you will get keys with no values  in those cases you will get errors if you use dictionary  key    whereas dictionary get  key    return otherwise   has no problems   Similarly  I would use    join list  as opposed to list 0  if you try to capture a single value from a list   hope it helps    Edit  Here is a practical example   Say  you are calling an API  which returns a JOSN file you need to parse  The first JSON looks like following     bids    id  16210506  submitdate   2011-10-16 15 53 25   submitdate f   10  16  2011 at 21 53 CEST   submitdate f2   p u0159ed 2 lety   submitdate ts  1318794805  users id   2674360   project id   1250499      The second JOSN is like this     bids    id  16210506  submitdate   2011-10-16 15 53 25   submitdate f   10  16  2011 at 21 53 CEST   submitdate f2   p u0159ed 2 lety   users id   2674360   project id   1250499      Note that the second JSON is missing the  submitdate ts  key  which is pretty normal in any data structure   So when you try to access the value of that key in a loop  can you call it with the following   for item in API call      submitdate ts   item  bids    submitdate ts     You could  but it will give you a traceback error for the second JSON line  because the key simply doesn t exist   The appropriate way of coding this  could be the following   for item in API call      submitdate ts   item get  bids     x   None   get  submitdate ts       x   None  is there to avoid the second level getting an error  Of course you can build in more fault tolerance into the code if you are doing scraping  Like first specifying a if condition

User · Answer

It allows you to provide a default value if the key is missing   dictionary get  bogus   default value    returns default value  whatever you choose it to be   whereas  dictionary  bogus     would raise a KeyError     If omitted  default value is None  such that  dictionary get  bogus       lt -- No default specified -- defaults to None   returns None just like   dictionary get  bogus   None    would

User · Answer

The purpose is that you can give a default value if the key is not found  which is very useful  dictionary get  Name   harry

User · Answer

What is the dict get   method    As already mentioned the get method contains an additional parameter which indicates the missing value  From the documentation   get key   default         Return the value for key if key is in the dictionary  else default  If default is not given  it defaults to None  so that this method never raises a KeyError    An example can be    gt  gt  gt  d    1 2 2 3   gt  gt  gt  d 1  2  gt  gt  gt  d get 1  2  gt  gt  gt  d get 3   gt  gt  gt  repr d get 3    None   gt  gt  gt  d get 3 1  1      Are there speed improvements anywhere    As mentioned here      It seems that all three approaches now exhibit similar performance  within about 10  of each other   more or less independent of the properties of the list of words    Earlier get was considerably slower  However now the speed is almost comparable along with the additional advantage of returning the default value  But to clear all our queries  we can test on a fairly large list  Note that the test includes looking up all the valid keys only   def getway d       for i in range 100           s   d get i   def lookup d       for i in range 100           s   d i    Now timing these two functions using timeit   gt  gt  gt  import timeit  gt  gt  gt  print timeit timeit  getway  i i for i in range 100      from   main   import getway    20 2124660015  gt  gt  gt  print timeit timeit  lookup  i i for i in range 100      from   main   import lookup    16 16223979   As we can see the lookup is faster than the get as there is no function lookup  This can be seen through dis   gt  gt  gt  def lookup d val           return d val        gt  gt  gt  def getway d val           return d get val        gt  gt  gt  dis dis getway    2           0 LOAD FAST                0  d                3 LOAD ATTR                0  get                6 LOAD FAST                1  val                9 CALL FUNCTION            1              12 RETURN VALUE          gt  gt  gt  dis dis lookup    2           0 LOAD FAST                0  d                3 LOAD FAST                1  val                6 BINARY SUBSCR                      7 RETURN VALUE        Where will it be useful    It will be useful whenever you want to provide a default value whenever you are looking up a dictionary  This reduces   if key in dic        val   dic key   else        val   def val   To a single line  val   dic get key def val      Where will it be NOT useful    Whenever you want to return a KeyError stating that the particular key is not available  Returning a default value also carries the risk that a particular default value may be a key too       Is it possible to have get like feature in dict  key      Yes  We need to implement the   missing   in a dict subclass   A sample program can be  class MyDict dict       def   missing   self  key           return None   A small demonstration can be   gt  gt  gt  my d   MyDict  1 2 2 3    gt  gt  gt  my d 1  2  gt  gt  gt  my d 3   gt  gt  gt  repr my d 3    None

User · Answer

get takes a second optional value   If the specified key does not exist in your dictionary  then this value will be returned   dictionary     Name    Harry    Age   17  dictionary get  Year    No available data    gt  gt   No available data    If you do not give the second parameter  None will be returned   If you use indexing as in dictionary  Year    nonexistent keys will raise KeyError

User · Answer

A gotcha to be aware of when using  get    If the dictionary contains the key used in the call to  get   and its value is None  the  get   method will return None even if a default value is supplied  For example  the following returns None  not  alt value  as may be expected  d     key   None  assert None is d get  key    alt value     get   s second value is only returned if the key supplied is NOT in the dictionary  not if the return value of that call is None

[python] Why dict.get(key) instead of dict[key]?

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