Conditional statement in a one line lambda function in python

Question

Apologies if this has been asked before  but I couldn t see it anywhere   Essentially I ve come across a scenario where i need to make use of an if statement inside a lambda function  What makes it difficult is that ideally it needs to be in a single line of code  if thats even possible    Normally  i would write this   T   250  if  T  gt  200       rate   200 exp -T  else      rate   400 exp -T   return  rate    However i need it to look like this   rate   lambda T    if  T gt 200   return 200 exp -T    else  return 400 exp -T     I realize the easier thing to do would to take the decision making outside of the lambda functions  and then have a separate lambda function for each case  but its not really suitable here  The lambda functions are stored in an array and accessed when needed  with each array element corresponding to a particular  rate  so having two separate rows for the same  rate  would mess things up  Any help would be greatly appreciated  or if its not possible  some confirmation from others would be nice

User · Answer

Use the exp1 if cond else exp2 syntax   rate   lambda T  200 exp -T  if T gt 200 else 400 exp -T    Note you don t use return in lambda expressions

User · Answer

I found I COULD use  if-then  statements in a lambda   For instance   eval op                lambda x y  eval y  if  eval x   0  else eval x         amp      lambda x y  0 if  eval x   0  else eval y         lt      lambda x y  1 if  eval x  lt eval y   else 0        gt      lambda x y  1 if  eval x  gt eval y   else 0

User · Answer

Yes  you can use the shorthand syntax for if statements   rate   lambda t    200   exp -t   if t  gt  200 else  400   exp -t     Note that you don t use explicit return statements inlambdas either

User · Answer

By the time you say rate   lambda whatever    you ve defeated the point of lambda and should just define a function  But  if you want a lambda  you can use  and  and  or   lambda T    T gt 200  and  200 exp -T   or  400 exp -T

User · Answer

In case you want to be lazier    syntax lambda x    false true  Condition    In your case   rate   lambda T     400 exp -T  200 exp -T   T gt 200

User · Answer

The right way to do this is simple   def rate T       if  T  gt  200           return 200 exp -T      else          return 400 exp -T    There is absolutely no advantage to using lambda here  The only thing lambda is good for is allowing you to create anonymous functions and use them in an expression  as opposed to a statement   If you immediately assign the lambda to a variable  it s no longer anonymous  and it s used in a statement  so you re just making your code less readable for no reason   The rate function defined this way can be stored in an array  passed around  called  etc  in exactly the same way a lambda function could  It ll be exactly the same  except a bit easier to debug  introspect  etc       From a comment      Well the function needed to fit in one line  which i didn t think you could do with a named function    I can t imagine any good reason why the function would ever need to fit in one line  But sure  you can do that with a named function  Try this in your interpreter    gt  gt  gt  def foo x   return x   1        Also these functions are stored as strings which are then evaluated using  eval  which i wasn t sure how to do with regular functions    Again  while it s hard to be 100  sure without any clue as to why why you re doing this  I m at least 99  sure that you have no reason or a bad reason for this  Almost any time you think you want to pass Python functions around as strings and call eval so you can use them  you actually just want to pass Python functions around as functions and use them as functions   But on the off chance that this really is what you need here  Just use exec instead of eval   You didn t mention which version of Python you re using  In 3 x  the exec function has the exact same signature as the eval function   exec my function string  my globals  my locals    In 2 7  exec is a statement  not a function   but you can still write it in the same syntax as in 3 x  as long as you don t try to assign the return value to anything  and it works   In earlier 2 x  before 2 6  I think   you have to do it like this instead   exec my function string in my globals  my locals

[python] Conditional statement in a one line lambda function in python?

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