Append strings via dataframes in for loop

Question

I have 2 snippets which are giving me different outputs. The first snippet gives me the desired output but I want to replicate the same output with the 2nd snippet.

Snippet 1:

Top= pd.DataFrame({'A':['Hello', 'World']})

insrt1= pd.DataFrame({'A': [f'Appended Item-1x']})

df=Top['A'].append(insrt1['A'])
print(df)

Out:

0               Hello
1               World
0    Appended Item-1x
Name: A, dtype: object

The 2nd snippet yields a different output due to the for loop. How can I achieve the same output as in snippet 1?

Snippet2

Top= pd.DataFrame({'A':['Hello', 'World']})

Frst=['1','2']
Scnd=['x','y']

d={}

for num1 in Frst:
    for num2 in Scnd:
        key = f'-OE{num1}{num2}-'
        insrt2 = pd.DataFrame({'A': [f'Appended Item-{num1}{num2}']})
        d[key] = insrt2

df=Top['A'].append(d['-OE1x-'])
print(df)

Out:

     0                 A
0  Hello               NaN
1  World               NaN
0    NaN  Appended Item-1x

Could you please provide an example of expected/desired output? — JPI93
– JPI93, Commented Oct 5, 2020 at 12:56
Desired output is that shown from snippet 1. I want to replicate it with snippet 2 — shoggananna
– shoggananna, Commented Oct 5, 2020 at 12:59
Are you looking to append rows where column A has values Appended Item-1x, ...1y, ...2x, ...2y? Please be specific when asking others for help. — JPI93
– JPI93, Commented Oct 5, 2020 at 13:04
No. I would like just 1 column with 3 rows consisting of ‘Hello’ first entry , ‘World’ second entry and ‘ Appended Item-1x’ third entry with name of column being ‘A’ — shoggananna
– shoggananna, Commented Oct 5, 2020 at 13:22

JPI93 · Accepted Answer · 2020-10-05 13:52:39Z

1

The issue that's causing undesired output is coming from the fact that you're attempting to append a full Pandas.DataFrame to a Pandas.Series with the statement

df=Top['A'].append(d['-OE1x-'])

If you change this line to:

df = Top.append(d['-OE1x-'])

df will look like this:

                  A
0             Hello
1             World
0  Appended Item-1x

You may want to pass ignore_index=True as an argument to your call to Pandas.DataFrame.append() so that the row containing Appended Item-1x is given a sequential index - i.e the original index of 0 is not included as this would result in two rows with index 0 in df.

e.g.

df = Top.append(d['-OE1x-'], ignore_index=True)

Will give you the following df:

                  A
0             Hello
1             World
2  Appended Item-1x

Alternative Solution

Since it seems that you don't actually make use of each Pandas.DataFrame in d outside of appending them as new rows to existing dataframes, it may be a good idea to refactor your code so that each entry in d looks like str: str instead of str: Pandas.DataFrame. Using your original code you could achieve this as follows:

import pandas as pd

Top = pd.DataFrame({'A':['Hello', 'World']})

Frst = ['1','2']
Scnd = ['x','y']

d = { f'-OE{num1}{num2}-': f'Appended Item-{num1}{num2}' for num1 in Frst for num2 in Scnd }

df = Top.append({'A': d['-OE1x-']}, ignore_index=True)

This will also provide the desired df:

                  A
0             Hello
1             World
2  Appended Item-1x

However unlike the previous answer provided (and your original code) it will be much less memory intensive as d is not being filled unnecessarily with instances of Pandas.DataFrame.

answered Oct 5, 2020 at 13:52

JPI93

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3 Comments

shoggananna Over a year ago

@JP193. Both answers you provide work for me. Now I have come across another problem, details of which I did not include in the original question. It is essentially the appending of another dataframe of numbers after 'Append Item-1x'.

shoggananna Over a year ago

here is a snippet of my real problem 'df=TOP.append({'A':d['-OE1x-']},ignore_index=True).append (df.loc[0].append more.............`

shoggananna Over a year ago

the error is cannot concatenate object of type '<class 'dict'>'; only Series and DataFrame objs are valid

sharathnatraj · Accepted Answer · 2020-10-05 13:48:53Z

0

I think just convert those lists to series and append should do it.

Code:

import pandas as pd
Top= pd.DataFrame({'A':['Hello', 'World']})
Frst=['1','2']
Scnd=['x','y']
l = (pd.Series(Frst), pd.Series(Scnd))
df = Top.A
for i in l:
    df = df.append(i)
print(df)

Ouput:

0    Hello
1    World
0        1
1        2
0        x
1        y
dtype: object

answered Oct 5, 2020 at 13:48

sharathnatraj

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