Python safe method to get value of nested dictionary

Question

I have a nested dictionary  Is there only one way to get values out safely   try      example dict  key1    key2   except KeyError      pass   Or maybe python has a method like get   for nested dictionary

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glom is a nice library that can into dotted queries too  In  1   from glom import glom  In  2   data     a     b     c    d      In  3   glom data   quot a b c quot   Out 3    d   A query failure has a nice stack trace  indicating the exact failure spot  In  4   glom data   quot a b foo quot   --------------------------------------------------------------------------- PathAccessError                           Traceback  most recent call last   lt ipython-input-4-2a3467493ac4 gt  in  lt module gt  ---- gt  1 glom data   quot a b foo quot       cache pypoetry virtualenvs neural-knapsack-dE7ihQtM-py3 8 lib python3 8 site-packages glom core py in glom target  spec    kwargs     2179     2180     if err  - gt  2181         raise err    2182     return ret    2183   PathAccessError  error raised while processing  details below   Target-spec trace  most recent last    - Target    a     b     c    d      - Spec   a b foo  glom core PathAccessError  could not access  foo   part 2 of Path  a    b    foo    got error  KeyError  foo    Safeguard with default  In  5   glom data   quot a b foo quot   default  quot spam quot   Out 5    spam   The beauty of glom is in the versatile spec parameter  For example  one can easily extract all first names from the following data  In  8   data                  quot people quot                       quot first name quot    quot Alice quot    quot last name quot    quot Adams quot                      quot first name quot    quot Bob quot    quot last name quot    quot Barker quot                            In  9   glom data    quot people quot     quot first name quot     Out 9     Alice    Bob    Read the glom docs for more examples

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Yet another function for the same thing  also returns a boolean to represent whether the key was found or not and handles some unexpected errors       json   json to extract value from if exists path   details detail first name             empty path represents root  returns a tuple  boolean  object          boolean   True if path exists  otherwise False         object   the object if path exists otherwise None      def get json value at path json  path None  default None        if not bool path           return True  json     if type json  is not dict           raise ValueError f json  json   path  path  not supported  json must be a dict       if type path  is not str and type path  is not list          raise ValueError f path format  path  not supported  path can be a list of strings like  x y z  or a string like x y z        if type path  is str          path   path strip      split          key   path 0      if key in json keys            return get json value at path json key   path 1    default      else          return False  default   example usage   my json     details      first name     holla    last name     holla    print get json value at path my json   details first name        print get json value at path my json   details phone              True   holla         False

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After seeing this for deeply getting attributes  I made the following to safely get nested dict values using dot notation  This works for me because my dicts are deserialized MongoDB objects  so I know the key names don t contain  s  Also  in my context  I can specify a falsy fallback value  None  that I don t have in my data  so I can avoid the try except pattern when calling the function   from functools import reduce   Python 3 def deepgetitem obj  item  fallback None          Steps through an item chain to get the ultimate value       If ultimate value or path to value does not exist  does not raise     an exception and instead returns  fallback         gt  gt  gt  d     snl final     about      icsd     icsd id   1          gt  gt  gt  deepgetitem d   snl final about  icsd icsd id       1      gt  gt  gt  deepgetitem d   snl final about  sandbox sbx id        gt  gt  gt              def getitem obj  name           try              return obj name          except  KeyError  TypeError               return fallback     return reduce getitem  item split       obj

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A simple class that can wrap a dict  and retrieve based on a key   class FindKey dict       def get self  path  default None           keys   path split              val   None          for key in keys              if val                  if isinstance val  list                       val    v get key  default  if v else None for v in val                  else                      val   val get key  default              else                  val   dict get self  key  default               if not val                  break          return val   For example   person     person    name    first   John     FindDict person  get  person name first         John    If the key doesn t exist  it returns None by default  You can override that using a default  key in the FindDict wrapper -- for example    FindDict person  default     get  person name last        doesn t exist  so

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By combining all of these answer here and small changes that I made  I think this function would be useful  its safe  quick  easily maintainable   def deep get dictionary  keys  default None       return reduce lambda d  key  d get key  default  if isinstance d  dict  else default  keys split       dictionary    Example     gt  gt  gt  from functools import reduce  gt  gt  gt  def deep get dictionary  keys  default None           return reduce lambda d  key  d get key  default  if isinstance d  dict  else default  keys split       dictionary       gt  gt  gt  person     person    name    first   John      gt  gt  gt  print  deep get person   person name first    John  gt  gt  gt  print  deep get person   person name lastname    None  gt  gt  gt  print  deep get person   person name lastname   default  No lastname    No lastname  gt  gt  gt

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Recursive method                 Example dict  foo      feature name    Sample Creator  gt  Contract Details  gt  Elements of the page      scenarios      scenario name    SC  CD  Elements of the page                     scenario status    failed                     scenario tags   None                    steps      duration   0                               name    I open application Stage and login by                                        SPT LOGIN and password SPT PWD                                status    untested                                 duration   0                               name    I open Sample Creator query page                                status    untested                                 duration   7 78166389465332                               name    I open application Stage and login by                                        SPT LOGIN and password SPT PWD                                status    passed                                 duration   3 985326051712036                               name    I open Sample Creator query page                                status    passed                                 duration   2 9063704013824463                               name    Enter value                                         X-2008-CON-007 X-2011-CON-016 in                                        textarea  project text area sleep  1                                status    passed                                 duration   4 4447715282440186                               name    I press on GET DATA                                status    passed                                 duration   1 1209557056427002                               name    Verify the top table on Contract Details                                status    passed                                 duration   3 8173601627349854                               name    I export contract details table by offset                                        x 100  y 150                                status    passed                                 duration   1 032956600189209                               name    Check data of                                        sc  cd elements of the page 1 and skip                                        cols None                                status    passed                                 duration   0 04593634605407715                               name    quot Verify  Number of Substances  column  quot                                       values                                status    passed                                 duration   0 10199904441833496                               name    Substance Sample Details bottom table                                        columns                                status    passed                                 duration   0 0009999275207519531                               name    Verify the Substance Sample Details                                        bottom table                                status    passed                                 duration   3 8558616638183594                               name    I export substance sample details table                                        by offset x 100  y 150                                status    passed                                 duration   1 0329277515411377                               name    Check data of                                        sc  cd elements of the page 2 and skip                                        cols None                                status    passed                                 duration   0 2879970073699951                               name    Click on AG-13369                                status    passed                                 duration   3 800830364227295                               name    I export substance sample details table                                        by offset x 100  y 150                                status    passed                                 duration   1 0169551372528076                               name    Check data of                                        sc  cd elements of the page 3 and skip                                        cols None                                status    passed                                 duration   1 7484464645385742                               name    Select all cells  table  2                                status    passed                                 duration   3 812828779220581                               name    I export substance sample details table                                        by offset x 100  y 150                                status    passed                                 duration   1 0029594898223877                               name    Check data of                                        sc  cd elements of the page 2 and skip                                        cols None                                status    passed                                 duration   1 6729373931884766                               name    Set window size x 800  y 600                                status    passed                                 duration   30 145705699920654                               name    All scrollers are placed on top 6 and far                                        left 8                                status    failed            feature name    Sample Creator  gt  Substance Sample History  gt  Elements of the                      page      scenarios      scenario name    SC  SSH  Elements of the page                     scenario status    passed                     scenario tags   None                    steps      duration   0                               name    I open application Stage and login by                                        SPT LOGIN and password SPT PWD                                status    untested                                 duration   0                               name    I open Sample Creator query page                                status    untested                                 duration   7 305850505828857                               name    I open application Stage and login by                                        SPT LOGIN and password SPT PWD                                status    passed                                 duration   3 500955104827881                               name    I open Sample Creator query page                                status    passed                                 duration   3 0419492721557617                               name    Enter value  NOA401800 SYN-NOA                                        A S4A482070C SYN-ISN-OLD                                        O S04A482167T S04A482190Y CSAA796564 CSCD106701                                        in textarea  id text area sleep  1                                status    passed                                 duration   49 567158460617065                               name    I press on GET DATA                                status    passed                                 duration   0 13904356956481934                               name    Open substance sample history                                status    passed                                 duration   1 1039845943450928                               name    Columns displayed                                status    passed                                 duration   3 881945848464966                               name    I export export parent table table by                                        offset x 100  y 150                                status    passed                                 duration   1 0334820747375488                               name    Check data of                                        sc  ssh elements of the page 1 and skip                                        cols None                                status    passed                                 duration   0 0319981575012207                               name    quot Title is  Additional Details for Marked  quot                                       quot Rows  quot                                status    passed                                 duration   0 08897256851196289                               name    Columns displayed  the same as in top                                        table                                 status    passed                                 duration   25 192569971084595                               name    Verify the content of the bottom table                                status    passed                                 duration   4 308935880661011                               name    I export                                        additional details for marked rows table                                        by offset x 100  y 150                                status    passed                                 duration   1 0089836120605469                               name    Check data of                                        sc  ssh elements of the page 1 and skip                                        cols None                                status    passed         Code  def get keys  dict  dict  prefix  list       prefix    list  dict keys        return prefix   def  loop elements elems list  prefix None  limit None       prefix   prefix or        limit   limit or 9     try          if len elems     0 and isinstance elems  list               for   in elems                  if isinstance    dict                       get keys    prefix                      for item in   values                             loop elements item  prefix  limit          return prefix  limit      except TypeError          return    gt  gt  gt goo    loop elements foo limit 9   gt  gt  gt goo   feature name    scenarios    scenario name    scenario status    scenario tags    steps    duration    name    status

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You can use pydash   import pydash as      get example dict   key1 key2   default  Default     https   pydash readthedocs io en latest api html

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A solution I ve used that is similar to the double get but with the additional ability to avoid a TypeError using if else logic       value   example dict  key1    key2   if example dict get  key1   and example dict  key1   get  key2   else default value   However  the more nested the dictionary the more cumbersome this becomes

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For nested dictionary JSON lookups  you can use dictor  pip install dictor  dict object         characters              Lonestar                  id   55923               role    renegade                items                      space winnebago                    leather jacket                                    Barfolomew                  id   55924               role    mawg                items                      peanut butter jar                    waggy tail                                    Dark Helmet                  id   99999               role    Good is dumb                items                      Shwartz                    helmet                                    Skroob                  id   12345               role    Spaceballs CEO                items                      luggage                                    to get Lonestar s items  simply provide a dot-separated path  ie  import json from dictor import dictor  with open  test json   as data       data   json load data   print dictor data   characters Lonestar items     gt  gt   u space winnebago   u leather jacket     you can provide fallback value in case the key isnt in path  theres tons more options you can do  like ignore letter casing and using other characters besides     as a path separator   https   github com perfecto25 dictor

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def safeget  dct    keys       if not isinstance  dct  dict   raise TypeError  quot Is not instance of dict quot       def foo dct   keys           if len keys     0  return dct         elif not isinstance  dct  dict   return None         else  return foo dct get keys 0   None    keys 1        return foo  dct    keys   assert safeget dict       dict   assert safeget dict     quot test quot      None assert safeget dict    quot a quot   1    quot b quot   2     quot a quot    quot d quot      None assert safeget dict    quot a quot   1    quot b quot   2     quot a quot      1 assert safeget   quot a quot    quot b quot    quot c quot   2    quot d quot  1    quot a quot    quot b quot    quot c quot      2

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A recursive solution  It s not the most efficient but I find it a bit more readable than the other examples and it doesn t rely on functools   def deep get d  keys       if not keys or d is None          return d     return deep get d get keys 0    keys 1      Example  d     meta     status    OK    status code   200   deep get d    meta    status code            gt  200 deep get d    garbage    status code         gt  None     A more polished version  def deep get d  keys  default None               Example          d     meta     status    OK    status code   200           deep get d    meta    status code                 gt  200         deep get d    garbage    status code              gt  None         deep get d    meta    garbage    default  -       gt   -              assert type keys  is list     if d is None          return default     if not keys          return d     return deep get d get keys 0    keys 1    default

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You could also use python reduce   def deep get dictionary   keys       return reduce lambda d  key  d get key  if d else None  keys  dictionary

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While the reduce approach is neat and short  I think a simple loop is easier to grok  I ve also included a default parameter   def deep get  dict  keys  default None       for key in keys          if isinstance  dict  dict                dict    dict get key  default          else              return default     return  dict   As an exercise to understand how the reduce one-liner worked  I did the following  But ultimately the loop approach seems more intuitive to me   def deep get  dict  keys  default None        def  reducer d  key           if isinstance d  dict               return d get key  default          return default      return reduce  reducer  keys   dict    Usage  nested     a     b     c   42     print deep get nested    a    b    print deep get nested    a    b    z    z    default  missing

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Since raising an key error if one of keys is missing is a reasonable thing to do  we can even not check for it and get it as single as that   def get dict d  kl     cur   d kl 0     return get dict cur  kl 1    if len kl   gt  1 else cur

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Little improvement to reduce approach to make it work with list  Also using data path as string divided by dots instead of array   def deep get dictionary  path       keys   path split          return reduce lambda d  key  d int key   if isinstance d  list  else d get key  if d else None  keys  dictionary

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You could use get twice   example dict get  key1       get  key2     This will return None if either key1 or key2 does not exist   Note that this could still raise an AttributeError if example dict  key1   exists but is not a dict  or a dict-like object with a get method   The try  except code you posted would raise a TypeError instead if example dict  key1   is unsubscriptable   Another difference is that the try   except short-circuits immediately after the first missing key  The chain of get calls does not     If you wish to preserve the syntax  example dict  key1    key2   but do not want it to ever raise KeyErrors  then you could use the Hasher recipe   class Hasher dict         https   stackoverflow com a 3405143 190597     def   missing   self  key           value   self key    type self            return value  example dict   Hasher   print example dict  key1         print example dict  key1    key2         print type example dict  key1    key2        lt class    main   Hasher  gt    Note that this returns an empty Hasher when a key is missing   Since Hasher is a subclass of dict you can use a Hasher in much the same way you could use a dict  All the same methods and syntax is available  Hashers just treat missing keys differently   You can convert a regular dict into a Hasher like this   hasher   Hasher example dict    and convert a Hasher to a regular dict just as easily   regular dict   dict hasher      Another alternative is to hide the ugliness in a helper function   def safeget dct   keys       for key in keys          try              dct   dct key          except KeyError              return None     return dct   So the rest of your code can stay relatively readable   safeget example dict   key1    key2

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I suggest you to try python-benedict  It is a dict subclass that provides keypath support and much more  Installation  pip install python-benedict from benedict import benedict  example dict   benedict example dict  keypath separator       now you can access nested values using keypath  val   example dict  key1 key2      using  get  method to avoid a possible KeyError  val   example dict get  key1 key2    or access nested values using keys list  val   example dict  key1    key2      using get to avoid a possible KeyError  val   example dict get   key1    key2      It is well tested and open-source on GitHub  https   github com fabiocaccamo python-benedict Note  I am the author of this project

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for a second level key retrieving  you can do this   key2 value    example dict get  key1   or     get  key2

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There are already lots of good answers but I have come up with a function called get similar to lodash get in JavaScript land that also supports reaching into lists by index  def get value  keys  default value   None           Useful for reaching into nested JSON like data     Inspired by JavaScript lodash get and Clojure get-in etc        if value is None or keys is None        return None   path   keys split      if isinstance keys  str  else keys   result   value   def valid index key         return re match     1-9  0-9    0-9      key  and int key   gt   0   def is dict like v         return hasattr v     getitem     and hasattr v     contains       for key in path        if isinstance result  list  and valid index key  and int key   lt  len result             result   result int key   if int key   lt  len result  else None       elif is dict like result  and key in result            result   result key        else            result   default value           break   return result  def test get      assert get None    foo       None   assert get   foo   1   None     None   assert get None  None     None   assert get   foo   1            foo   1    assert get   foo   1     foo       1   assert get   foo   1     bar       None   assert get   foo   1     bar     the default       the default    assert get   foo     bar    hello       foo    bar        hello    assert get   foo     bar    hello      foo bar       hello    assert get   foo      bar    hello       foo 0 bar       hello    assert get   foo      bar    hello       foo 1      None   assert get   foo      bar    hello       foo 1 bar      None   assert get   foo    bar     1       bar    assert get   foo    bar     2      None

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Building up on Yoav s answer  an even safer approach   def deep get dictionary   keys       return reduce lambda d  key  d get key  None  if isinstance d  dict  else None  keys  dictionary

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An adaptation of unutbu s answer that I found useful in my own code   example dict setdefaut  key1       get  key2     It generates a dictionary entry for key1 if it does not have that key already so that you avoid the KeyError  If you want to end up a nested dictionary that includes that key pairing anyway like I did  this seems like the easiest solution

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I little changed this answer  I added checking if we re using list with numbers  So now we can use it whichever way  deep get allTemp   0       or deep get getMinimalTemp   0  minimalTemperatureKey   26  etc  def deep get  dict  keys  default None       def  reducer d  key           if isinstance d  dict               return d get key  default          if isinstance d  list               return d key  if len d   gt  0 else default         return default     return reduce  reducer  keys   dict

[python] Python safe method to get value of nested dictionary

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