append multiple values for one key in a dictionary

Question

I am new to python and I have a list of years and values for each year  What I want to do is check if the year already exists in a dictionary and if it does  append the value to that list of values for the specific key   So for instance  I have a list of years and have one value for each year   2010   2   2009   4   1989   8   2009   7     What I want to do is populate a dictionary with the years as keys and those single digit numbers as values  However  if I have 2009 listed twice  I want to append that second value to my list of values in that dictionary  so I want     2010  2   2009  4  7   1989  8     Right now I have the following     d   dict     years          get 2 column list of years and values   for line in list          year   line 0         value   line 1     for line in list        if year in d keys              d value  append value        else            d value    value           d year    year

User · Answer

You would be best off using collections defaultdict  added in Python 2 5   This allows you to specify the default object type of a missing key  such as a list    So instead of creating a key if it doesn t exist first and then appending to the value of the key  you cut out the middle-man and just directly append to non-existing keys to get the desired result   A quick example using your data    gt  gt  gt  from collections import defaultdict  gt  gt  gt  data     2010  2    2009  4    1989  8    2009  7    gt  gt  gt  d   defaultdict list   gt  gt  gt  d defaultdict  lt type  list  gt        gt  gt  gt  for year  month in data          d year  append month        gt  gt  gt  d defaultdict  lt type  list  gt    2009   4  7   2010   2   1989   8      This way you don t have to worry about whether you ve seen a digit associated with a year or not  You just append and forget  knowing that a missing key will always be a list  If a key already exists  then it will just be appended to

User · Answer

If I can rephrase your question  what you want is a dictionary with the years as keys and an array for each year containing a list of values associated with that year  right  Here s how I d do it   years dict   dict    for line in list      if line 0  in years dict            append the new number to the existing array at this slot         years dict line 0   append line 1       else            create a new array in this slot         years dict line 0      line 1     What you should end up with in years dict is a dictionary that looks like the following          2010    2        2009    4 7        1989    8      In general  it s poor programming practice to create  parallel arrays   where items are implicitly associated with each other by having the same index rather than being proper children of a container that encompasses them both

User · Answer

Here is an alternative way of doing this using the not in operator      define an empty dict years dict   dict    for line in list        here define what key is  for example      key   line 0        check if key is already present in dict     if key not in years dict          years dict key             append some value      years dict key  append some value

User · Answer

You can use setdefault    for line in list        d setdefault year      append value    This works because setdefault returns the list as well as setting it on the dictionary  and because a list is mutable  appending to the version returned by setdefault is the same as appending it to the version inside the dictionary itself  If that makes any sense

User · Answer

If you want a  almost  one-liner    from collections import deque  d      deque  d setdefault year      append value  for year  value in source of data   maxlen 0    Using dict setdefault  you can encapsulate the idea of  check if the key already exists and make a new list if not  into a single call  This allows you to write a generator expression which is consumed by deque as efficiently as possible since the queue length is set to zero  The deque will be discarded immediately and the result will be in d   This is something I just did for fun  I don t recommend using it  There is a time and a place to consume arbitrary iterables through a deque  and this is definitely not it

User · Answer

It s easier if you get these values into a list of tuples  To do this  you can use list slicing and the zip function   data in    2010 2 2009 4 1989 8 2009 7  data pairs   zip data in   2  data in 1  2     Zip takes an arbitrary number of lists  in this case the even and odd entries of data in  and puts them together into a tuple   Now we can use the setdefault method   data dict      for x in data pairs      data dict setdefault x 0      append x 1     setdefault takes a key and a default value  and returns either associated value  or if there is no current value  the default value  In this case  we will either get an empty or populated list  which we then append the current value to

User · Answer

d          import list of year value pairs  for year value in mylist      try          d year  append value      except KeyError          d year     value    The Python way - it is easier to receive forgiveness than ask permission

[python] append multiple values for one key in a dictionary

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