Translating Ruby. How do I do this in Python?

Question

I want to find a particular element in a list like this:

my_list.find { |e| e == 'find this' }

I know in Python I can do:

[e for e in my_list if e == 'find this']

But this returns a list as opposed to the element I want.

I know how I can do this by iterating through the list, but I want to know if there's a more concise way of doing it.

In your simplified case, the result would be the same as `'find this' if 'find this' in my_list else None`, as you are using equality here. — Martijn Pieters, Jul 25 '14 at 15:24
It's not necessary to say "Edit:" or notate that something changed. Stack Overflow keeps an edit history so we can see how things have changed if necessary. — the Tin Man, Jul 25 '14 at 15:28

score 2 · Answer 1 · answered Jul 25 '14 at 15:25

2

You can find the first matching element with a generator expression and next():

match = next((e for e in my_list if e == 'find this'), None)

This sets match to None if no elements matched.

Because a generator expression is used, only enough elements in my_list are inspected to find the first match, after which the search is stopped.

The sample search you gave is rather pointless, of course, as you are basically using simple equality. For more complex attribute searches it makes more sense.

answered Jul 25 '14 at 15:25

Martijn Pieters

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I suspect the OP meant `find_index` in which case @miku's answer is correct :) – Jon Clements Jul 25 '14 at 15:28
@JonClements: maybe, maybe not. We'll see! – Martijn Pieters Jul 25 '14 at 15:29
Mind you, have remembered depending on the Ruby version, there's the `Enumerable` interface so `.find` would correlate with this answer (using the default parameter to return `None` is similar to the `nil` result Ruby returns from `.find`) :p – Jon Clements Jul 25 '14 at 15:37
Indeed, this is equal to [Ruby's Enumerable#find](http://ruby-doc.org/core-2.1.2/Enumerable.html#method-i-find). It yields the first matching element or `nil`, not the index. – Daniël Knippers Jul 25 '14 at 17:24
@DaniëlKnippers: then `next()` with a generator expression and a default does exactly that. – Martijn Pieters Jul 25 '14 at 17:25
@MartijnPieters Yes, I was referring to your answer so we agree :-) – Daniël Knippers Jul 25 '14 at 17:26

miku · Answer 2 · 2014-07-25T15:31:49.110

0

You could use list.index, which

returns the index in the list of the first item whose value is x. It is an error if there is no such item.

>>> l = ["1", "se", "uz", "tz", "find this", "that"]
>>> l.index("find this") 
4

>>> l[l.index("find this")]
'find this'

>>> l[l.index("find that")]
...
ValueError: 'find that' is not in list

If all your app does is searching for exact keys, you could also use a dictionary (called Hash in Ruby, I guess) instead - to reduce lookup times from O(n) to amortized O(1).

edited Jul 25 '14 at 15:31

answered Jul 25 '14 at 15:26

miku

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We just call it a hash in Ruby. It's called many things in different languages. – the Tin Man Jul 25 '14 at 15:29
Just corrected it this moment, after skimming the Ruby docs, but thanks for noticing. – miku Jul 25 '14 at 15:30

Translating Ruby. How do I do this in Python?

2 Answers2