Question: This is an observation from Most pythonic way to concatenate pandas cells with conditions I am not able to understand why third solution one takes more memory compared to first one.  If I don't sample the third solution does not give runtime error, clearly something is weird  To emulate large dataframe I tried to resample, but never expected to run into this kind of error   Background Pretty self explanatory, one line, looks pythonic df['city'] + (df['city'] == 'paris')*('_' + df['arr'].astype(str))  s = """city,arr,final_target paris,11,paris_11 paris,12,paris_12 dallas,22,dallas miami,15,miami paris,16,paris_16""" import pandas as pd import io df = pd.read_csv(io.StringIO(s)).sample(1000000, replace=True) df  Speeds %%timeit df['city'] + (df['city'] == 'paris')*('_' + df['arr'].astype(str)) # 877 ms ± 19.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  %%timeit df['final_target'] = np.where(df['city'].eq('paris'), df['city'] + '_' + df['arr'].astype(str), df['city']) # 874 ms ± 19.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  If I dont sample, there is no error and output also match exactly Error(Updated)(Only happens when I sample from dataframe) %%timeit df['final_target'] = df['city'] df.loc[df['city'] == 'paris', 'final_target'] += '_' + df['arr'].astype(str)  MemoryError: Unable to allocate 892. GiB for an array with shape (119671145392,) and data type int64  For smaller input(sample size 100) we get different error, telling a problem due to different sizes, but whats up with memory allocations and sampling? ValueError: cannot reindex from a duplicate axis --------------------------------------------------------------------------- ValueError Traceback (most recent call last) <ipython-input-5-57c5b10090b2> in <module> 1 df['final_target'] = df['city'] ----> 2 df.loc[df['city'] == 'paris', 'final_target'] += '_' + df['arr'].astype(str) ~/anaconda3/lib/python3.8/site-packages/pandas/core/ops/methods.py in f(self, other) 99 # we are updating inplace so we want to ignore is_copy 100 self._update_inplace( --> 101 result.reindex_like(self, copy=False), verify_is_copy=False 102 ) 103  I rerun them from scratch each time Update This is part of what I figured s = """city,arr,final_target paris,11,paris_11 paris,12,paris_12 dallas,22,dallas miami,15,miami paris,16,paris_16""" import pandas as pd import io df = pd.read_csv(io.StringIO(s)).sample(10, replace=True) df   city arr final_target 1 paris 12 paris_12 0 paris 11 paris_11 2 dallas 22 dallas 2 dallas 22 dallas 3 miami 15 miami 3 miami 15 miami 2 dallas 22 dallas 1 paris 12 paris_12 0 paris 11 paris_11 3 miami 15 miami   Indices are repeated when sampled with replacement  So resetting the indices resolved the problem even if df.arr and df.loc have essentially different sizes or replacing with df.loc[df['city'] == 'paris', 'arr'].astype(str) will solve it. Just as 2e0byo pointed out.  Still can someone explain how .loc works and also explosion of memory When indices have duplicates in them and don't match?!  

Answer: @2e0byo hit the nail on the head saying pandas' algorithm is "inefficient" in this case. As far as .loc, it's not really doing anything remarkable. Its use here is analogous to indexing a numpy array with a boolean array of the same shape, with an added dict-key-like access to a specific column - that is, df['city'] == 'paris' is itself a dataframe, with the same number of rows and the same indexes as df, with a single column of boolean values. df.loc[df['city'] == 'paris'] then gives a dataframe consisting of only the rows that are true in df['city'] == 'paris' (that have 'paris' in the 'city' column). Adding the additional argument 'final_target' then just returns only the 'final_target' column of those rows, instead of all three (and because it only has one column, it's technically a Series object - the same goes for df['arr']). The memory explosion happens when pandas actually tries to add the two Series. As @2e0byo pointed out, it has to reshape the Series to do this, and it does this by calling the first Series' align() method. During the align operation, the function pandas.core.reshape.merge.get_join_indexers() calls pandas._libs.join.full_outer_join() (line 155) with three arguments: left, right, and max_groups (point of clarification: these are their names inside the function full_outer_join). left and right are integer arrays containing the indexes of the two Series objects (the values in the index column), and max_groups is the maximum number of unique elements in either left or right (in our case, that's five, corresponding to the five original rows in s). full_outer_join immediately turns and calls pandas._libs.algos.groupsort_indexer() (line 194), once with left and max_groups as arguments and once with right and max_groups. groupsort_indexer returns two arrays - generically, indexer and counts (for the invocation with left, these are called left_sorter and left_count, and correspondingly for right). counts has length max_groups + 1, and each element (excepting the first one, which is unused) contains the count of how many times the corresponding index group appears in the input array. So for our case, with max_groups = 5, the count arrays have shape (6,), and elements 1-5 represent the number of times the 5 unique index values appear in left and right. The other array, indexer, is constructed so that indexing the original input array with it returns all the elements grouped in ascending order - hence "sorter." After having done this for both left and right, full_outer_join chops up the two sorters and strings them up across from each other. full_outer_join returns two arrays of the same size, left_idx and right_idx - these are the arrays that get really big and throw the error. The order of elements in the sorters determines the order they appear in the final two output arrays, and the count arrays determine how often each one appears. Since left goes first, its elements stay together - in left_idx, the first left_count[1] elements in left_sorter are repeated right_count[1] times each (aaabbbccc...). At the same place in right_idx, the first right_count[1] elements are repeated in a row left_count[1] times (abcabcabc...). (Conveniently, since the 0 row in s is a 'paris' row, left_count[1] and right_count[1] are always equal, so you get x amount of repeats x amount of times to start off). Then the next left_count[2] elements of left_sorter are repeated right_count[2] times, and so on... If any of the counts elements are zero, the corresponding spots in the idx arrays are filled with -1, to be masked later (as in, right_count[i] = 0 means elements in right_idx are -1, and vice versa - this is always the case for left_count[3] and left_count[4], because rows 2 and 3 in s are non-'paris'). In the end, the _idx arrays have an amount of elements equal to N_elements, which can be calculated as follows: left_nonzero = (left_count[1:] != 0) right_nonzero = (right_count[1:] != 0) left_repeats = left_count[1:]*left_nonzero + np.ones(len(left_counts)-1)*(1 - left_nonzero) right_repeats = right_count[1:]*right_nonzero + np.ones(len(right_counts)-1)*(1 - right_nonzero) N_elements = sum(left_repeats*right_repeats)  The corresponding elements of the count arrays are multiplied together (with all the zeros replaced with ones), and added together to get N_elements. You can see this figure grows pretty quickly (O(n^2)). For an original dataframe with 1,000,000 sampled rows, each one appearing about equally, then the count arrays look something like: left_count = array([0, 2e5, 2e5, 0, 0, 2e5]) right_count = array([0, 2e5, 2e5, 2e5, 2e5, 2e5])  for a total length of about 1.2e11. In general for an initial sample N (df = pd.read_csv(io.StringIO(s)).sample(N, replace=True)), the final size is approximately 0.12*N**2 An Example It's probably helpful to look at a small example to see what full_outer_join and groupsort_indexer are trying to do when they make those ginormous arrays. We'll start with a small sample of only 10 rows, and follow the various arrays to the final output, left_idx and right_idx. We'll start by defining the initial dataframe: df = pd.read_csv(io.StringIO(s)).sample(10, replace=True) df['final_target'] = df['city'] # this line doesn't change much, but meh  which looks like:  city arr final_target 3 miami 15 miami 1 paris 11 paris 0 paris 12 paris 0 paris 12 paris 0 paris 12 paris 1 paris 11 paris 2 dallas 22 dallas 3 miami 15 miami 2 dallas 22 dallas 4 paris 16 paris  df.loc[df['city'] == 'paris', 'final_target'] looks like: 1 paris 0 paris 0 paris 0 paris 1 paris 4 paris  and df['arr'].astype(str): 3 15 1 11 0 12 0 12 0 12 1 11 2 22 3 15 2 22 4 16  Then, in the call to full_outer_join, our arguments look like: left = array([1,0,0,0,1,4]) # indexes of df.loc[df['city'] == 'paris', 'final_target'] right = array([3,1,0,0,0,1,2,3,2,4]) # indexes of df['arr'].astype(str) max_groups = 5 # the max number of unique elements in either left or right  The function call groupsort_indexer(left, max_groups) returns the following two arrays: left_sorter = array([1, 2, 3, 0, 4, 5]) left_count = array([0, 3, 2, 0, 0, 1])  left_count holds the number of appearances of each unique value in left - the first element is unused, but then there a 3 zeros, 2 ones, 0 twos, 0 threes, and 1 four in left. left_sorter is such that left[left_sorter] = array([0, 0, 0, 1, 1, 4]) - all in order. Now right: groupsort_indexer(right, max_groups) returns right_sorter = array([2, 3, 4, 1, 5, 6, 8, 0, 7, 9]) right_count = array([0, 3, 2, 2, 2, 1])  Once again, right_count contains the number of times each count appears: the unused first element, and then 3 zeros, 2 ones, 2 twos, 2 threes, and 1 four (note that elements 1, 2, and 5 of both count arrays are the same: these are the rows in s with 'city' = 'paris'). Also, right[right_sorter] = array([0, 0, 0, 1, 1, 2, 2, 3, 3, 4]) With both count arrays calculated, we can calculate what size the idx arrays will be (a bit simpler with actual numbers than with the formula above): N_total = 3*3 + 2*2 + 2 + 2 + 1*1 = 18  3 is element 1 for both counts arrays, so we can expect something like [1,1,1,2,2,2,3,3,3] to start left_idx, since [1,2,3] starts left_sorter, and [2,3,4,2,3,4,2,3,4] to start right_idx, since right_sorter begins with [2,3,4]. Then we have twos, so [0,0,4,4] for left_idx and [1,5,1,5] for right_idx. Then left_count has two zeros, and right_count has two twos, so next go 4 -1's in left_idx and the next four elements in right_sorter go into right_idx: [6,8,0,7]. Both count's finish with a one, so one each of the last elements in the sorters go in the idx: 5 for left_idx and 9 for right_idx, leaving: left_idx = array([1, 1, 1, 2, 2, 2, 3, 3, 3, 0, 0, 4, 4,-1, -1, -1, -1, 5]) right_idx = array([2, 3, 4, 2, 3, 4, 2, 3, 4, 1, 5, 1, 5, 6, 8, 0 , 7, 9])  which is indeed 18 elements. With both index arrays the same shape, pandas can construct two Series of the same shape from our original ones to do any operations it needs to, and then it can mask these arrays to get back sorted indexes. Using a simple boolean filter to look at how we just sorted left and right with the outputs, we get: left[left_idx[left_idx != -1]] = array([0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 4]) right[right_idx[right_idx != -1]] = array([0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 3, 3, 4])  After going back up through all the function calls and modules, the result of the addition at this point is: 0 paris_12 0 paris_12 0 paris_12 0 paris_12 0 paris_12 0 paris_12 0 paris_12 0 paris_12 0 paris_12 1 paris_11 1 paris_11 1 paris_11 1 paris_11 2 NaN 2 NaN 3 NaN 3 NaN 4 paris_16  which is result in the line result = op(self, other) in pandas.core.generic.NDFrame._inplace_method (line 11066), with op = pandas.core.series.Series.__add__ and self and other the two Series from before that we're adding. So, as far as I can tell, pandas basically tries to perform the operation for every combination of identically-indexed rows (like, any and all rows with index 1 in the first Series should be operated with all rows index 1 in the other Series). If one of the Series has indexes that the other one doesn't, those rows get masked out. It just so happens in this case that every row with the same index is identical. It works (albeit redundantly) as long as you don't need to do anything in place - the trouble for the small dataframes arises after this when pandas tries to reindex this result back into the shape of the original dataframe df. The split (the line that smaller dataframes make it past, but larger ones don't) is that line result = op(self, other) from above. Later in the same function (called, note, _inplace_method), the program exits at self._update_inplace(result.reindex_like(self, copy=False), verify_is_copy=False). It tries to reindex result so it looks like self, so it can replace self with result (self is the original Series, the first one in the addition, df.loc[df['city'] == 'paris', 'final_target']). And this is where the smaller case fails, because, obviously, result has a bunch of repeated indexes, and pandas doesn't want to lose any information when it deletes some of them. One Last Thing It's probably worth mentioning that this behaviour isn't particular to the addition operation here. It happens any time you try an arithmetic operation on two large dataframes with a lot of repeated indexes - for example, try just defining a second dataframe the exact same way as the first, df2 = pd.read_csv(io.StringIO(s)).sample(1000000, replace=True), and then try running df.arr*df2.arr. You'll get the same memory error. Interestingly, logical and comparison operators have protections against doing this - they require identical indexes, and check for it before calling their operator method.  I did all my stuff in pandas 1.2.4, python 3.7.10, but I've given links to the pandas Github, which is currently in version 1.3.3. As far as I can tell, the differences don't affect the results.