No Multiline Lambda in Python: Why Not

No Multiline Lambda in Python: Why not?

Look at the following:

map(multilambda x:
      y=x+1
      return y
   , [1,2,3])

Is this a lambda returning (y, [1,2,3]) (thus map only gets one parameter, resulting in an error)? Or does it return y? Or is it a syntax error, because the comma on the new line is misplaced? How would Python know what you want?

Within the parens, indentation doesn't matter to python, so you can't unambiguously work with multilines.

This is just a simple one, there's probably more examples.

How to write python lambda with multiple lines?

Use def instead.

def d(x):
    if x:
        return 1
    else:
        return 2

All python functions are first order objects (they can be passed as arguments), lambda is just a convenient way to make short ones. In general, you are better off using a normal function definition if it becomes anything beyond one line of simple code.

Even then, in fact, if you are assigning it to a name, I would always use def over lambda. lambda is really only a good idea when defining short key functions, for use with sorted(), for example, as they can be placed inline into the function call.

Note that, in your case, a ternary operator would do the job (lambda x: 1 if x else 2), but I'm presuming this is a simplified case.

(As a code golf note, this could also be done in less code as lambda x: bool(x)+1 - of course, that's highly unreadable and a bad idea.)

Python multiline lambda

You can define your lambda on multiple lines if you put the expression in parentheses. This creates an implied line continuation, causing newlines to be ignored up to the closing parenthesis.

>>> func = lambda a,b: (
...     b - a if a <= b else
...     a*b
... )
>>>
>>> func(23, 42)
19

You can also explicitly use the line continuation character "\", but this is not the approach preferred by the Python style guide. (Not that binding lambdas to names are a good idea to begin with, in fairness...)

>>> func = lambda a,b: \
...     b - a if a <= b else \
...     a*b
>>>
>>> func(23, 42)
19

Of course, you can only have expressions inside your lambda, and not statements. So proper if blocks and returns and the like are still impossible.

---

Additionally, it may not be necessary to use lambdas here at all, because unlike C# (prior to the recent v. 7.0), Python is capable of nesting full functions:

>>> def method(x,y):
...     def func(a,b):
...             if a <= b:
...                     return b - a
...             return a * b
...     return func(x,y)
...
>>> method(23, 42)
19

Lambda not in function doesn't work for more than one word in Python

You could use str.contains:

items = ("boxer","boxers","sock","socks")

import numpy as np
df["category1"] = np.where(df['product_name'].str.contains('|'.join(items)),
                           np.nan,  # value is True
                           'Other') # value if False

output:

     product_name category1
0      blue shirt     Other
1   medium boxers       nan
2    red jackets      Other
3       blue sock       nan

why are my python lambda functions not working?

.append() is an in place update, and returns None, so your current generator is creating a list of None which can't be compared by sorted().

That aside, you don't need to keep track of your newly split items with map() as it's a generator that is being fed into sorted().

Try:

>>> my_list = ["a,a", "b,b", "c,c", "a,b"]
>>> sorted(map(lambda i: i.split(","), my_list))
[['a', 'a'], ['a', 'b'], ['b', 'b'], ['c', 'c']]

Or using a generator expression (my preferred method):

>>> my_list = ["a,a", "b,b", "c,c", "a,b"]
>>> sorted(i.split(",") for i in my_list)
[['a', 'a'], ['a', 'b'], ['b', 'b'], ['c', 'c']]

Why doesn't lambda function accept parameter in bond with sorted()

The key parameter of sorted is a function which will be applied to the elements in order to compare them to each other.
Therefore, we only pass the function itself without calling it.

I suggest you read the documentation first next time :)
https://docs.python.org/3/library/functions.html#sorted

Does Lambda function not required?

There should be no difference in the speed of both functions ref: lambda is slower than function call in python, why (spoiler alert, it's not)

A lambda is just a function created with a single expression and no name.

normalize trained data with tensorflow

The best way to explain I think is to see a simpler example:

list(map(lambda n: n * 3, [1, 2, 3, 4, 5]))
>> [3, 6, 9, 12, 15]

In this case you're applying to each (because of map) element n of the list [1, 2, 3, 4, 5] the same operation n * 3.

In your specific case it's a bit more complicated because you have two variables. What happens is that for each (x, y) pair of your train_data you are leaving y as is, and applying the normalization to x.

Update, more info on what is a lambda.

This site offers great examples and a good definition of lambda used inside a map function, like in your case:

A lambda expression is a way of creating a little function inline,
without all the syntax of a def. …

The code of the lambda is typically a single expression without
variables or if-statements, and does not use "return". Lambda is
perfect where you have a short computation to write inline.

---

Related Topics