column dataframe to int

# convert all columns of DataFrame
df = df.apply(pd.to_numeric) # convert all columns of DataFrame

# convert just columns "a" and "b"
df[["a", "b"]] = df[["a", "b"]].apply(pd.to_numeric)

# convert Series
my_series = pd.to_numeric(my_series)

# convert column "a" of a DataFrame
df["a"] = pd.to_numeric(df["a"])

# convert Series
my_series = pd.to_numeric(my_series)
#
# convert column "a" of a DataFrame
df["a"] = pd.to_numeric(df["a"])