Assignment inside lambda expression in Python

Question

I have a list of objects and I want to remove all objects that are empty except for one  using filter and a lambda expression   For example if the input is      Object name      Object name  fake name    Object name           then the output should be      Object name      Object name  fake name      Is there a way to add an assignment to a lambda expression  For example     flag   True  input    Object name      Object name  fake name    Object name       output   filter       lambda o   flag or bool o name   flag   flag and bool o name   0        input

User · Answer

No  you cannot put an assignment inside a lambda because of its own definition  If you work using functional programming  then you must assume that your values are not mutable   One solution would be the following code   output   lambda l  name     if l                    else   l  0       output  l 1    name   if l  0   name    name                else output  l 1    name   if l  0   name                      else   l  0       output  l 1    name

User · Answer

The assignment expression operator    added in Python 3 8 supports assignment inside of lambda expressions  This operator can only appear within a parenthesized        bracketed        or braced       expression for syntactic reasons  For example  we will be able to write the following   import sys say hello   lambda        message     Hello world       sys stdout write message     n     -1  say hello     In Python 2  it was possible to perform local assignments as a side effect of list comprehensions   import sys say hello   lambda         None for message in   Hello world         sys stdout write message     n     -1  say hello     However  it s not possible to use either of these in your example because your variable flag is in an outer scope  not the lambda s scope  This doesn t have to do with lambda  it s the general behaviour in Python 2  Python 3 lets you get around this with the nonlocal keyword inside of defs  but nonlocal can t be used inside lambdas   There s a workaround  see below   but while we re on the topic       In some cases you can use this to do everything inside of a lambda    lambda          def          for sys in    import    sys            for math in    import    math             for sub in  lambda  vals  None          for fun in  lambda  vals  vals -1            for echo in  lambda  vals  sub              sys stdout write u    join map unicode  vals     u  n              for Cylinder in  type  Cylinder    object    dict                init     lambda self  radius  height  sub                  setattr self   radius   radius                   setattr self   height   height                 volume   property lambda self  fun                    def  for top area in  math pi   self radius    2                     self height   top area               for main in  lambda  sub                loop  for factor in  1  2  3  if sub                    def                      for my radius  my height in   10   factor  20   factor                       for my cylinder in  Cylinder my radius  my height                      echo u A cylinder with a radius of   1fcm and a height                        u of   1fcm has a volume of   1fcm                             my radius  my height  my cylinder volume              main            A cylinder with a radius of 10 0cm and a height of 20 0cm has a volume of 6283 2cm      A cylinder with a radius of 20 0cm and a height of 40 0cm has a volume of 50265 5cm      A cylinder with a radius of 30 0cm and a height of 60 0cm has a volume of 169646 0cm      Please don t        back to your original example  though you can t perform assignments to the flag variable in the outer scope  you can use functions to modify the previously-assigned value   For example  flag could be an object whose  value we set  using setattr   flag   Object value True  input    Object name      Object name  fake name    Object name       output   filter lambda o        flag value or bool o name       setattr flag   value   flag value and bool o name     0   input     Object name      Object name  fake name      If we wanted to fit the above theme  we could use a list comprehension instead of setattr        None for flag value in  bool o name        But really  in serious code you should always use a regular function definition instead of a lambda if you re going to be doing outer assignment   flag   Object value True  def not empty except first o       result   flag value or bool o name      flag value   flag value and bool o name      return result input    Object name      Object name  fake name    Object name       output   filter not empty except first  input

User · Answer

TL DR  When using functional idioms it s better to write functional code  As many people have pointed out  in Python lambdas assignment is not allowed   In general when using functional idioms your better off thinking in a functional manner which means wherever possible no side effects and no assignments   Here is functional solution which uses a lambda   I ve assigned the lambda to fn for clarity  and because it got a little long-ish    from operator import add from itertools import ifilter  ifilterfalse fn   lambda l  pred  add list ifilter pred  iter l      ifilterfalse pred  iter l   next     objs    Object name      Object name  fake name    Object name      fn objs  lambda o  o name          You can also make this deal with iterators rather than lists by changing things around a little   You also have some different imports   from itertools import chain  islice  ifilter  ifilterfalse fn   lambda l  pred  chain ifilter pred  iter l    islice ifilterfalse pred  iter l    1     You can always reoganize the code to reduce the length of the statements

User · Answer

Normal assignment     is not possible inside a lambda expression  although it is possible to perform various tricks with setattr and friends     Solving your problem  however  is actually quite simple   input    Object name      Object name  fake name    Object name      output   filter      lambda o   seen set            not  not o and o in  seen or  seen add o        input         which will give you   Object Object name      name  fake name      As you can see  it s keeping the first blank instance instead of the last   If you need the last instead  reverse the list going in to filter  and reverse the list coming out of filter   output   filter      lambda o   seen set            not  not o and o in  seen or  seen add o        input   -1          -1    which will give you   Object name  fake name    Object name        One thing to be aware of  in order for this to work with arbitrary objects  those objects must properly implement   eq   and   hash   as explained here

User · Answer

Kind of a messy workaround  but assignment in lambdas is illegal anyway  so it doesn t really matter  You can use the builtin exec   function to run assignment from inside the lambda  such as this example    gt  gt  gt  val Traceback  most recent call last     File   lt pyshell 31 gt    line 1  in  lt module gt      val NameError  name  val  is not defined  gt  gt  gt  d   lambda  exec  val True   globals     gt  gt  gt  d    gt  gt  gt  val True

User · Answer

UPDATE    o for d in      for o in lst if o name       or d setdefault     o     o    or using filter and lambda   flag      filter lambda o  bool o name  or flag setdefault     o     o  lst    Previous Answer  OK  are you stuck on using filter and lambda   It seems like this would be better served with a dictionary comprehension    o name   o for o in input  values     I think the reason that Python doesn t allow assignment in a lambda is similar to why it doesn t allow assignment in a comprehension and that s got something to do with the fact that these things are evaluated on the C side and thus can give us an increase in speed  At least that s my impression after reading one of Guido s essays   My guess is this would also go against the philosophy of having one right way of doing any one thing in Python

User · Answer

first   you dont need to use a local assigment for your job  just check the above answer  second  its simple to use locals   and globals   to got the variables table and then change the value  check this sample code   print  locals     setitem    x    Hillo       x  -1    if you need to change the add a global variable to your environ  try to replace locals   with globals    python s list comp is cool but most of the triditional project dont accept this like flask      hope it could help

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You can use a bind function to use a pseudo multi-statement lambda   Then you can use a wrapper class for a Flag to enable assignment   bind   lambda x  f  lambda y  y   f x   class Flag object       def   init   self  value           self value   value      def set self  value           self value   value         return value  input    Object name      Object name  fake name    Object name      flag   Flag True  output   filter              lambda o                    bind flag value  lambda orig flag value                  bind flag set flag value and bool o name    lambda                    bind orig flag value or bool o name                   input

User · Answer

If instead of flag   True we can do an import instead  then I think this meets the criteria    gt  gt  gt  from itertools import count  gt  gt  gt  a     hello        world                bob    gt  gt  gt  filter lambda L  j count    L or not next j   a    hello        world    bob     Or maybe the filter is better written as    gt  gt  gt  filter lambda L  blank count count 1   L or next blank count     1  a    Or  just for a simple boolean  without any imports   filter lambda L  use blank iter  True    L or next use blank  False   a

User · Answer

You cannot really maintain state in a filter lambda expression  unless abusing the global namespace   You can however achieve something similar using the accumulated result being passed around in a reduce   expression    gt  gt  gt  f   lambda a  b   a append b  or a  if  b not in a  else a  gt  gt  gt  input     foo   u     bar            x    gt  gt  gt  reduce f  input        foo   u     bar    x    gt  gt  gt     You can  of course  tweak the condition a bit  In this case it filters out duplicates  but you can also use a count      for example  to only restrict empty strings   Needless to say  you can do this but you really shouldn t      Lastly  you can do anything in pure Python lambda  http   vanderwijk info blog pure-lambda-calculus-python

User · Answer

If you need a lambda to remember state between calls  I would recommend either a function declared in the local namespace or a class with an overloaded   call    Now that all my cautions against what you are trying to do is out of the way  we can get to an actual answer to your query   If you really need to have your lambda to have some memory between calls  you can define it like   f   lambda o  ns     flag  True    ns  flag   or o name  ns   setitem    flag   ns  flag   and o name   0    Then you just need to pass f to filter    If you really need to  you can get back the value of flag with the following   f   defaults   0   flag     Alternatively  you can modify the global namespace by modifying the result of globals    Unfortunately  you cannot modify the local namespace in the same way as modifying the result of locals   doesn t affect the local namespace

User · Answer

The pythonic way to track state during iteration is with generators  The itertools way is quite hard to understand IMHO and trying to hack lambdas to do this is plain silly  I d try   def keep last empty input       last   None     for item in iter input           if item name  yield item         else  last   item     if last is not None  yield last  output   list keep last empty input     Overall  readability trumps compactness every time

User · Answer

There s no need to use a lambda  when you can remove all the null ones  and put one back if the input size changes   input    Object name      Object name  fake name    Object name       output    x for x in input if x name  if len input     len output        output append Object name

[python] Assignment inside lambda expression in Python

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