Python: How to copy elements from list that start with a certain letter into new list or remove elements from list that do not start with letter

Question

I am getting a list from an excel file using pandas.

start_path = r'C:\scratch\\'
File = 'test.xlsx'
import pandas as pd 
mylist = []
df = pd.read_excel(start_path + File, sheet_name='GIS')
mylist = df['Column A'].tolist()

List:

mylist = ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56', 'ABC']

My goal is to then create a new list from this list, only with the elements that start with LB. So the new list would then be:

newlist = ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56']

Or just remove all elements from a list that do not start with 'LB' (thus removing ABC from the list).

newlist = [str(x for x in mylist if "LB" in x)]

I tried the above and this just spits out:

['<generator object <genexpr> at 0x0000024B5B8F62C8>']

I also have tried the following:

approved = ['LB']
mylist[:] = [str(x for x in mylist if any(sub in x for sub in approved))]

This gets the same generator object message as before.

I feel like this is really simple but cannot figure it out.

Answer 1

You can use str.startswith in list-comprehension:

mylist = ["LB-52/LP-7", "LB-53/LI-5", "LB-54/LP-8", "LB-55", "LB-56", "ABC"]

newlist = [value for value in mylist if value.startswith("LB")]
print(newlist)

Prints:

['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56']

---

You can remove str() in newlist = [str(x for x in mylist if "LB" in x)] but this will leave values such as xxxLBxxx (LB is inside the string)

Answer 2

newlist = [x for x in mylist if x[0:2]=="LB"]

You can also use slicing and also check on desired indices with slicing

Answer 3

In general, pandas tends to do the same job faster than python. Therefore you should try to do most of your computation or filtering in pandas before "moving over" to python.

mylist = df.loc[df['Column A'].str.startswith("LB"), 'Column A'].tolist()
mylist
>>> ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56']

Answer 4

mylist = ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56', 'ABC']
new_list = [*filter(lambda x: x.startswith('LB'), mylist)]
# ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56']

Answer 5

This code works

newlist = ['LB-52/LP-7', 'LB-53/LI-5', 'LB-54/LP-8', 'LB-55', 'LB-56']
mylist=   [ch for ch in newlist if ch[0:2]=="LB"]