How to use format() on pandas dataframe variable

Question

I have the followings pandas dataframes

phreatic_level_l2n1_28w_df.head()
       Fecha    Hora    PORVL2N1  # PORVLxNx column change their name in each data frame
0   2012-01-12  01:37:47    0.65
1   2012-01-12  02:37:45    0.65
2   2012-01-12  03:37:50    0.64
3   2012-01-12  04:37:44    0.63
4   2012-01-12  05:37:45    0.61

And so, successively until have 25 data frames of type phreatic_level_l24n2_28w_df

.
.
.
phreatic_level_l24n2_28w_df.head()
       Fecha    Hora    PORVL24N2 # PORVLxNx column change their name in each data frame
0   2018-01-12  01:07:28    1.31
1   2018-01-12  02:07:28    1.31
2   2018-01-12  03:07:29    1.31
3   2018-01-12  04:07:27    1.31
4   2018-01-12  05:07:27    1.31

My objective is to iterate each record ( all data frames) to apply the following process

for i in range(1,25):
    if (i==2):
        # We turn to datetime the Fecha column values 
        phreatic_level_l{}n{}_28w_df['Fecha'].format(i,i-1) = pd.to_datetime(phreatic_level_l'{}'n'{}'_28w_df['Fecha'].format(i,i-1))
    .
    .
    # And so, successively until have 25 data frames

But I have the following error, due to format() function, it should be applied on strings only and not to any variable name.

  File "<ipython-input-72-1f6ad7811399>", line 5
    phreatic_level_l{}n{}_28w_df['Fecha'].format(i,i-1) = pd.to_datetime(phreatic_level_l'{}'n'{}'_28w_df['Fecha'].format(i,i-1))
                    ^
SyntaxError: invalid syntax

Am I understanding the problem correctly is that you have the 25 dataframes in 25 separate variables and need to iterate over them? If so there's some ugly things you can do with globals() or vars(), but what you should really do is just make a list of the 25 DFs and iterate over that — AdamKG
– AdamKG, Commented Oct 16, 2018 at 22:21

aydow · Accepted Answer · 2018-10-16 22:28:01Z

1

str.format works on strings. You're trying to use it on a variable name.

You could place your DataFrames in a dict and then reference them by string.

dfs = {
    'phreatic_level_l1n0_28w_df': phreatic_level_l1n0_28w_df,
    'phreatic_level_l2n1_28w_df': phreatic_level_l1n0_28w_df,
    'phreatic_level_l3n2_28w_df': phreatic_level_l1n0_28w_df,
    ...
}

for name, df in dfs.items():
    df = pd.to_datetime(df['Fecha'])

You can also access specific DataFrames like so dfs['phreatic_level_l3n2_28w_df'].

Alternatively, you can store them in a list and iterate over them

dfs = [
    phreatic_level_l1n0_28w_df,
    phreatic_level_l2n1_28w_df,
    phreatic_level_l3n2_28w_df,
    ...
]

for df in dfs:
    df = pd.to_datetime(df['Fecha'])

If you've stored them in order by the variable names you can access them in a much less verbose way, i.e. dfs[0].

Finally, check out this is a great tutorial on str.format

answered Oct 16, 2018 at 22:28

aydow

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3 Comments

bgarcial Over a year ago

Hi @aydow, I've could iterate over my data frames dictionary according to your tip. According to this, I've explained better my objective, which generates the average per day value of the PORVLxNx . At the top of my question (UPDATING MY QUESTION title in forward) I've explained my goal with all this process. Just in case, can you check my question again please? :)

aydow Over a year ago

hi @bgarcial, this looks like a totally separate issue. please ask a different question to address your new issue and i (or someone else) will look to answer there :) in the meantime, if my answer has helped you, please consider up voting :)

bgarcial Over a year ago

Hi @aydow, I've created other question issue, just in case if you have the opportunity to check it :) stackoverflow.com/questions/52860165/…

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