Python Nested Loops And Else Clause in List Comprehension

Question

I have two lists:

lst = ['Go','Go','Go','Go','Dont Go!','Go','Go','Go','Dont Go!','Go'] 
tls = ['G', 'Go', 'Go1', 'Go2', 'Go3']

I need to check for each element in tls if it exists in lst then to output that element, otherwise to output NaN.

I need output to be list like:

['Go', 'Go', 'Go', 'Go', nan, 'Go', 'Go', 'Go', nan, 'Go']

I managed to achieve this by using nested for loops:

ml = []
for t in tls:
    for l in lst:
        if t in lst:
            if t !=l:
                ml.append(np.nan)
            else:
                ml.append(t)
        else:
            pass

Is it possible to add else clause in this list comprehension in order to achieve the same result?

[t for t in tls for l in lst if t ==l]

The output for this list comprehension:

['Go', 'Go', 'Go', 'Go', 'Go', 'Go', 'Go', 'Go']

Expected output:

['Go', 'Go', 'Go', 'Go', nan, 'Go', 'Go', 'Go', nan, 'Go']

Thank You.

Answer 1

Here's how you could do so using a list comprehension. For a better performance you could take a set of tls reducing in this way the complexity of checking for membership to O(1) :

lst = ['Go','Go','Go','Go','Dont Go!','Go','Go','Go','Dont Go!','Go'] 
tls = set(['G', 'Go', 'Go1', 'Go2', 'Go3'])

[i if i in tls else float('nan') for i in lst]
#['Go', 'Go', 'Go', 'Go', nan, 'Go', 'Go', 'Go', nan, 'Go']

Answer 2

If I understand correctly, you're trying to check for each element in lst whether that element matches with any element in the list tls . Then try this :

[t if t in tls else np.nan for t in lst]

Output :

['Go', 'Go', 'Go', 'Go', nan, 'Go', 'Go', 'Go', nan, 'Go']

Answer 3

我不确定我是否正确地理解了您的任务，但是可以简单地实现所需的结果

[t if t in tls else np.nan for t in lst]

Answer 4

Python math module also introduce math.nan from Python 3.5 .

A floating-point “not a number” (NaN) value. Equivalent to the output of float('nan').

Try this,

>>> from math import nan
>>> 
>>> lst = ['Go','Go','Go','Go','Dont Go!','Go','Go','Go','Dont Go!','Go']
>>> tls = ['G', 'Go', 'Go1', 'Go2', 'Go3']
>>> [i if i in tls else nan for i in lst]
['Go', 'Go', 'Go', 'Go', nan, 'Go', 'Go', 'Go', nan, 'Go']